Reservoir characterisation of đăng nhập 188bet middle miocene Ca Voi Xanh isolated carbonate platform

Beata von Schnurbein, Nguyen Xuan Phong

Esso Australia Pty. Ltd.

ExxonMobil Exploration & Production Vietnam Ltd. 3ExxonMobil Production Germany GmbH 4ExxonMobil Research Qatar

Petrovietnam Exploration Production Corporation Email: christian.j.strohmenger@exxonmobil.com

Summary

đăng nhập 188bet Ca Voi Xanh field is located offshore Vietnam in đăng nhập 188bet southern Song Hong basin. Tertiary carbonates of Middle Miocene (Langhian and Serravallian) age form an isolated carbonate platform on top of đăng nhập 188bet Triton Horst structural high. Shallow water corals and large and small benthic foraminifera are đăng nhập 188bet main constituents of đăng nhập 188bet older Langhian carbonates, whereas đăng nhập 188bet overlying Serravallian carbonates are dominated by deeper water coralline red algae (rhodolith) and large benthic foraminifera (LBF).

Sequence stratigraphic interpretation is based on detailed sedimentolological core description tied towell-log character. A sequence stratigraphic framework was established for đăng nhập 188bet Serravallian carbonates, displaying three third-order depositional sequences (Ser1, Ser2, and Ser3).

Langhian carbonates can be described by one lithofacies: coral-LBF-foraminifera grainstone-packstone/rudstone. Depending on đăng nhập 188bet presence of mud in đăng nhập 188bet samples, two lithofacies types are characteristic for đăng nhập 188bet Serravallian: LBF-rhodolith packstone to mud-lean packstone and rhodolith-LBF grainstone to mud-lean packstone.

Two well-developed exposure surfaces can be identified on top of đăng nhập 188bet Langhian (Serravallian sequence boundary Ser1_SB) and on top of đăng nhập 188bet Serravallian (Tortonian sequence boundary Tor1_SB). Serravallian carbonates show an overall shallowing-upward trend from more horizontally-oriented coralline red algae (encrusted and bored pavements/hardgrounds) at đăng nhập 188bet lower part of đăng nhập 188bet section to large, roundish, irregular rhodoliths towards đăng nhập 188bet upper part of đăng nhập 188bet section. Petrographic thin section, stable isotope (oxygen and carbon), and fluid inclusion analyses confirm a freshwater (vadose and phreatic) diagenetic overprint of đăng nhập 188bet carbonates below đăng nhập 188bet exposure surfaces (sequence boundaries).

Partial dolomitisation observed at Well-2 and Well-3 might be related to CO2 and hydrocarbon charging, causing secondary leaching of đăng nhập 188bet Mg-rich coralline red algae and LBFs; leading to an enrichment of đăng nhập 188bet water with Mg-ions and thus favouring downward dolomitisation of đăng nhập 188bet Serravallian carbonates.

A reproducible reservoir rock type (RRT) scheme was developed for đăng nhập 188bet described carbonates, using a combination of depositional environment, diagenetic overprint, and reservoir parameters (porosity and permeability). đăng nhập 188bet Langhian is characterised by two RRTs, depending on đăng nhập 188bet degree of cementation (RRT-3L) and dissolution (RRT-5L). đăng nhập 188bet Serravallian RRTs are separated into dominantly packstone (RRT1) and dominantly grainstone textures (RRT-2, RRT-3, RRT-5, and RRT-6). RRTs with grainstone texture show varying degrees of cementation (RRT-2 and RRT-3) and dissolution (RRT-5 and RRT-6). Early diagenetic dolomitisation (RRT-4) preferentially effects packstone (RRT-1), but also grainstone textures (RRT-3 and RRT-5). đăng nhập 188bet vertical and lateral distribution of RRTs, supported by seismically-derived paleo-reconstruction of đăng nhập 188bet carbonate platform, adequately describes đăng nhập 188bet reservoir.

đăng nhập 188bet geologic (static) model consists of both, matrix and non-matrix components. Seismically-derived probablitiy maps for each of đăng nhập 188bet six Seravallian RRTs, based on 3D seismic paleo-structure mapping, were used as input to đăng nhập 188bet matrix component of đăng nhập 188bet geologic model. Different karst geometries and features consistent with known hydro-geologic processes are identified in đăng nhập 188bet seismic discontinuity (variance) cube and are used to interpret karst regions with different degrees and/or types of karst/fractures. These karst regions are used to populate đăng nhập 188bet non- matrix component of đăng nhập 188bet geologic model along đăng nhập 188bet identified two major sequence boundaries (Ser1_SB and Tor1_SB).

Key words: Reservoir characterisation, carbonate platform, Ca Voi Xanh field.

1. Introduction

đăng nhập 188bet Ca Voi Xanh field is located offshore Vietnam in đăng nhập 188bet southern part of đăng nhập 188bet Song Hong basin, between đăng nhập 188bet Qiongdongnan basin in đăng nhập 188bet north and đăng nhập 188bet Phu Khanh basin in đăng nhập 188bet south [1, 2]. đăng nhập 188bet greater East Sea area has a relatively simple tectonic history. In đăng nhập 188bet Eocene to Early Oligocene, extension (rifting) was initiated, followed by movements along đăng nhập 188bet Red River/East Vietnam Boundary Fault Zone approximately 45 million years ago. đăng nhập 188bet Red River Fault is a major left-lateral strike-slip fault system caused by đăng nhập 188bet collision of đăng nhập 188bet India and Asia tectonic plates. Motion on đăng nhập 188bet Red River Fault System continued into đăng nhập 188bet Early Miocene, and marine conditions, triggering carbonate platform growth, were established East of đăng nhập 188bet fault zone on top of structural highs. Carbonate growth was widespread on structural highs offshore Central Vietnam throughout đăng nhập 188bet Early and Middle Miocene. Regional uplift of đăng nhập 188bet mainland resulted in increased influx of siliciclastics from đăng nhập 188bet West; leading to stressed carbonate growth and finally to đăng nhập 188bet drowning of đăng nhập 188bet Tri Ton horst carbonate platform during đăng nhập 188bet Late Miocene [1, 2].

đăng nhập 188bet generalised stratigraphy of đăng nhập 188bet region is shown on Figure 1a. Middle Miocene age carbonate platforms of đăng nhập 188bet Da Nang Formation nucleated on older, remnant syn-rift highs. đăng nhập 188bet Ca Voi Xanh platform is an isolated carbonate platform that was established on đăng nhập 188bet Tri Ton horst structural high (Figure 1b). đăng nhập 188bet horst is a positive feature separated in đăng nhập 188bet West from land by a deep trough that trapped time-equivalent clastics and allowed deposition of carbonates on đăng nhập 188bet Tri Ton horst structural high. Carbonate deposition ended when clastics filled đăng nhập 188bet trough and prograded over đăng nhập 188bet carbonate platform.

2. Sequence stratigraphy

đăng nhập 188bet complete carbonate interval at Ca Voi Xanh and south of Ca Voi Xanh was penetrated by two wells, where it is approximately 600 to 800m thick. đăng nhập 188bet well penetrations in đăng nhập 188bet main Ca Voi Xanh platform (Well-2, Well-3, and Well-4) only penetrated đăng nhập 188bet upper portion (less than 200m) of đăng nhập 188bet carbonate platform. Well-3 and Well-4 were cored across đăng nhập 188bet Serravalian sequence boundary Ser1_ SB into đăng nhập 188bet Langhian section. đăng nhập 188bet core obtained from đăng nhập 188bet Well-2 only covers đăng nhập 188bet Serravallian interval (Figure 2).

Sequence stratigraphic correlation between Well-2, Well-3, and Well-4 is based on observed facies distribution, biostratigraphic interpretation, and well-log character. Conventional core was recovered from all three wells, in addition to sidewall cores from Well-2 and Well-4. A sequence stratigraphic framework was established for đăng nhập 188bet Serravallian carbonates. đăng nhập 188bet Serravallian interval can be described by one third-order composite sequence [3], comprising three third-order depositional sequences: Ser1, Ser2, and Ser3 (Figure 2). Four major regionally correlative surfaces can be carried across đăng nhập 188bet Ca Voi Xanh platform: Serravallian sequence boundary Ser1_SB (identified on well-logs, core, and seismic), Serravallian maximum flooding surface Ser2_MFS (identified on well-logs and core only), Tortonian sequence boundary Tor1_SB (identified on well-logs, core, and seismic), and Tortonian flooding surface/sequence boundary Tor1_FSSB (identified on well-logs and core only), merging with đăng nhập 188bet Tor1_SB at Well-2 and Well-3. đăng nhập 188bet Ser2_MFS (Serravallian third- order composite maximum flooding surface) correlates with a zone of bioturbated hardgrounds with platy corals observed in core material of all three wells and is closely associated with an elevated gamma ray response (Figure 2). Generally, above đăng nhập 188bet third-order composite Ser2_MFS, Serravallian carbonates show decreasing (lower) GR-log values, compared to Serravallian carbonates below đăng nhập 188bet Ser2_MFS (Figure 2).

Langhian carbonates, directly underlying đăng nhập 188bet Serravallian third-order sequence boundary Ser1_SB display root marks filled with detrital quartz grains, indicative of an exposure surface [4] (Figure 3). Serravallian carbonates show a cemented interval of varying thickness, interpreted to be đăng nhập 188bet result of exposure, directly below đăng nhập 188bet third-order Tortonian sequence boundary Tor1_SB (Figure 4a). Glossifungites burrows, as well as fluid inclusion (brackish to fresh water inclusions) and stable isotopes (typical “inverted J” trend of oxygen and carbon isotopes [5, 6]; Figure 4b) analyses support đăng nhập 188bet interpretation of an exposure surface, related to a sea-level lowstand at đăng nhập 188bet end of Serravallian time.

đăng nhập 188bet Tor1_SB at Well-4 (top of cemented zone, approximately 10m below đăng nhập 188bet top carbonate) is correlated with đăng nhập 188bet top carbonate picks at Well-2 and Well-3 (Figure 2). đăng nhập 188bet interpreted Tor1_FSSB, marking đăng nhập 188bet top carbonate at Well-4, merges with đăng nhập 188bet Tor1_SB in đăng nhập 188bet other wells (absence of uppermost, approximately 10m thick carbonate succession; Figure 2).

Figure 2 shows đăng nhập 188bet above described integrated stratigraphic framework on a North-South oriented well cross-section.

3. Environment of deposition

đăng nhập 188bet Environment of Deposition (EOD) interpretation is based on đăng nhập 188bet integrated sequence stratigraphic framework, đăng nhập 188bet antecedent topography, and biostratigraphic analyses, mostly done on large benthic foraminifera (LBF) and coralline red algae (rhodoliths).

Langhian-age deposits are interpreted to inherit a broad central platform morphology based on core- interpreted facies and seismic facies analysis across đăng nhập 188bet platform. Serravallian deposits are interpreted to occupy a wide facies belt associated with a central platform high. đăng nhập 188bet depositional style changes between Langhian (rimmed platform) and Serravallian (carbonate ramp) time. Both, Langhian and Serravallian carbonates were exposed during sea-level lowstands at đăng nhập 188bet end of Langhian and Serravallian times. 

During đăng nhập 188bet Langhian, đăng nhập 188bet common presence of hermatypic coral rudstones, interlayered with packstones, and grainstones, rich in large benthic (Miogypsina) and, predomminantly, small benthic foraminifera, including miliolids, soritids, and alveolinids point to a warm, shallow marine, inner shelf environment of deposition [7]. Coral- rich patch reefs can be found forming today in many platform-interior, shallow, warm, well-lit settings [8, 9]. Langhian carbonates represent a typical photozoan association of organisms [9].

đăng nhập 188bet Serravallian third-order composite sequence is interpreted to represent a deeper open marine, proximal to distal middle ramp environment [11]. đăng nhập 188bet carbonates are rich in large benthic foraminifera (LBF) like Cycloclypeus, Katacycloclypeus, Amphistegina, and Lepidocyclina, coralline red algae with encrusting foraminifera (Acervulina inhaerens), and minor small benthic and planktonic foraminifera. In contrast to đăng nhập 188bet Langhian, Serravallian carbonates represent a typical heterozoan association of organisms [10].

Coralline red algae (Corallinales, Halidiales and Sporolithales, Rhodophyta) are common and are represented by members of đăng nhập 188bet subfamilies Neogoniolithoideae, Mastophoroideae, Melobesioideae, and Lithophylloideae (family Corallinaceae), and members of đăng nhập 188bet families Hapalidiaceae and Sporolithaceae. Six non- geniculate genera (seven species) of coralline red algae were recognised. Neogoniolithoideae: Neogoniolithon and Spongites; Mastophoroideae: Lithoporella; Lithophylloideae: Lithophyllum; Melobesioideae: Mesophyllum and undetermined melobesioids; Sporolithaceae: Sporolithon [11].

Coralline red algae occur as encrusting thalli floating in đăng nhập 188bet sediment matrix, rhodoliths, as well as thallial fragments. Rhodolith shapes range from sub-spheroidal

to flat shapes. Inner and outer parts of đăng nhập 188bet rhodolith do not show different growth-forms. Encrusting growth-forms are very common in đăng nhập 188bet outer parts of đăng nhập 188bet rhodoliths. Rare warty morphologies are also present. Encrusting foraminifera are represented by acervulinids which, together with đăng nhập 188bet coralline red algae, make up rhodoliths and đăng nhập 188bet encrusting coralline framework (i.e., crustose pavement). Acervulinids can be dominating and often alternate with coralline red algae making superimposed successions of coralline thalli/acervulinid shells [11].

Coralline algae can be common in both tropical and non-tropical settings, though they seem to be more prevalent as facies dominating components in đăng nhập 188bet latter. In đăng nhập 188bet studied thin sections, reef-building corals and dasycladalean green algae are missing; suggesting that these deposits may represent a tropical/warm temperate water transitional area. Non-tropical heterozoan carbonates do not form carbonate platforms with steep, wave-resistant rims, but can accumulate hydrodynamically as carbonate ramps.

đăng nhập 188bet coralline red algal and larger foraminiferal assemblages suggest deposition in environments between (below) fair- weather wave base (FWWB) and storm wave base (SWB), or what is referred to as đăng nhập 188bet middle ramp setting [12 - 15]. This setting was further differentiated based on gradient ranging from: (1) đăng nhập 188bet proximal middle ramp or shallowest part close to đăng nhập 188bet FWWB and characterised by higher coralline taxonomic diversity and abundant larger nummulitid foraminifera to (2) đăng nhập 188bet distal middle ramp, representing đăng nhập 188bet transition to đăng nhập 188bet outer ramp, characterised by lower coralline taxonomic diversity, dominated by melobesioids, and larger orbitoidiform foraminifera [11].

đăng nhập 188bet dominance of Neogoniolithoideae in shallow water settings with an increase in đăng nhập 188bet presence of Melobesioideae with depth has been observed in many areas including đăng nhập 188bet Hawaiian Islands [16], Ryukyu Islands [17 - 19], Papua New Guinea [20], Gulf of Mexico [21, 22], Rio de Janeiro [23], and South Pacific reefs [22]. đăng nhập 188bet sporolithacean Sporolithon occurs from tropical to temperate areas [25 - 27] from sea- level down to 80m water depth [16, 25, 28 - 30].

Middle Miocene sediments with abundant rhodoliths (composed of Lithothamnion and Sporolithon) from đăng nhập 188bet Queensland Plateau and Marion Plateau are believed to be deposited in water depths between 30 and 80m [14, 15, 31].

On đăng nhập 188bet Kikai-jima shelf (Central Ryukyu Islands, Japan), at water depths of 61 to 105m, sandy and gravelly carbonate sediments are characterised by macroid  pavements

[32]. Macroids, ranging in size from ca. 25 to 130mm in diameter, are spheroidal and sub-spheroidal in shape and consist mainly of đăng nhập 188bet encrusting foraminifera Acervulina inhaerens and subordinate thin encrusting and lumpy coralline algae. đăng nhập 188bet macroid pavements are forming under low sedimentation rates and occasional movement due to current action. Similar environmental characteristics can be argued for đăng nhập 188bet studied material. đăng nhập 188bet occurrence of đăng nhập 188bet ichnotaxa Gastrochaenolites and Entobia confirm a low sedimentation rate of đăng nhập 188bet setting.

Crustose pavements consist of coralline algal bindstone made up of thin encrusting coralline thalli, superimposed onto each other. đăng nhập 188bet framework points to low turbulence interpreted to be in đăng nhập 188bet distal portion of đăng nhập 188bet middle ramp. Somewhat contradicting, đăng nhập 188bet pavement is also characterised by Neogoniolithon.

Also Rhodolith beds are sites of biodiversity, they are habitats hampering đăng nhập 188bet occurrence of larger foraminifera. đăng nhập 188bet dominance of acervulinids and melobesioids can be indicative of a middle ramp, below 50m water depth (a similar setting in which đăng nhập 188bet Kikai-jima macroids grow; low sedimentation rate, occasional overturning [32]). However, đăng nhập 188bet occurrence of Neogoniolithon suggests a shallower setting. đăng nhập 188bet occurrence of Neogoniolithon both in đăng nhập 188bet crustose pavement and in rhodolith beds could correlate to two settings, located not very far from each other. Within đăng nhập 188bet proximal to distal middle ramp, đăng nhập 188bet crustose pavement occur in a low energy, deeper water area, whilst đăng nhập 188bet rhodolith beds in an occasionally turbulent, somewhat shallower setting.

đăng nhập 188bet depositional system of đăng nhập 188bet analysed thin-section material is interpreted as a ramp. đăng nhập 188bet environment of deposition ranges from a proximal middle ramp, relatively shallow-water setting below đăng nhập 188bet FWWB, to a distal middle ramp, realtively deeper-water setting with occasional high water motion (storms) causing rhodolith overturning. An inner ramp setting is ruled out due to (1) đăng nhập 188bet absence of very shallow-water coralline taxonomic assemblage and components such as hermatypic corals, abundant larger porcellaneous foraminifera and dasycladaleans green algae, and (2) đăng nhập 188bet abundant occurrence of orbitoidiform larger foraminifera (typical of middle-outer ramp environment).

A summary of đăng nhập 188bet rhodoliths identified and their general paleogeographic setting [11] are:

Proximal Middle Ramp

- Dominated by Mastophoroideae (Lithoporella) and Neogoniolithoideae (Neogoniolithon, Spongites)

- Subordinate corallines such as Melobesioideae (Mesophyllum), Lithophylloideae (Lithophyllum) and Sporolithaceae (Sporolithon)

- Higher coralline taxonomic diversity

- Occurrence of nummulitid larger foraminifera

- Occurrence of larger porcellaneous foraminifera (Orbitolites, Neoplanorbulinella) Distal Middle Ramp

- Dominated by Melobesioideae (Mesophyllum)

- Subordinate corallines such as Mastophoroideae (Lithoporella), Neogoniolithoideae (Neogoniolithon, Spongites), and undetermined melobesioids

- Lower coralline taxonomic diversity

- Bioerosion by boring sponges (Entobia) and bivalves (Gastrochaenolites)

A likely average paleo-water depth for đăng nhập 188bet Serravallian sediments is interpreted to be between 50 and 80m (distal middle ramp) below Ser2_MFS, and < 80m, shallowing- upward to approximately 30m above Ser2_MFS; towards đăng nhập 188bet top of đăng nhập 188bet reservoir (Tor1_SB).

đăng nhập 188bet Serravallian carbonate section is overlain by Tortonian, deeper marine, outer ramp, marls or claystones with > 90% planktonic foraminifera.

4. Diagenesis

Detailed analysis of core and thin section material has led to an understanding of đăng nhập 188bet paragenetic sequence for đăng nhập 188bet Serravallian carbonate reservoir that highlights đăng nhập 188bet importance of diagenetic overprint rather than primary deposition on reservoir parameter trends.

Alteration of Ca Voi Xanh carbonate sediments began in đăng nhập 188bet marine realm soon after deposition (both in Langhian and Serravallian times). Early seafloor alteration of sediments is apparent in most thin sections in đăng nhập 188bet form of grain micritisation and micritic envelope formation. These alterations occur through đăng nhập 188bet activity of bacteria and algae borings into grains. Thin isopachous fibrous, marine cements (neomorphosed aragonite cement) are present in some Ca Voi Xanh thin sections, but are volumetrically insignificant. More common are isopachous bladed, marine cements (neomorphosed high-Mg calcite cement). Neither grain micritisation nor seafloor cementation has a major impact on reservoir quality. Notably missing in đăng nhập 188bet Serravallian sequence are large numbers of original aragonitic allochems (green algae, molluscs, and corals). Importantly, đăng nhập 188bet Serravallian fossil assemblage is rich in Mg-calcite, as both đăng nhập 188bet rhodoliths [10] and đăng nhập 188bet large benthic foraminifers such as Amphistegina [33] are composed of this mineral.

Meteoric diagenesis (both vadose and phreatic) follows early sediment alteration (Figure 4). There are two important aspects to this alteration with respect to reservoir impact: dissolution of allochems and precipitation of calcite cements. Most dissolution is fabric- selective with allochems such as coral fragments, benthic foraminifera, and red algal grains commonly leached. Partial leaching of grains results in microporous textures, whereas complete leaching results in mold formation. Non-fabric-selective dissolution is commonly observed below interpreted exposure surfaces [4]. đăng nhập 188bet dissolution/ cementation below sequence boundaries is interpreted to be meteoric as it is associated with meteoric cement fabrics (cryptocrystalline meniscus, circumgranular, dog tooth, and blocky cements), and occurs prior to compaction. Fluid inclusion (brackish to fresh water inclusions) and stable isotopes (carbon and oxygen), following đăng nhập 188bet meteoric or “inverted J” trend [5, 6], demonstrate that these cements have been formed by meteoric early diagenesis.

Most of đăng nhập 188bet data points follow đăng nhập 188bet burial trend whereby variations in 13C are minor compared to đăng nhập 188bet overall variations in 18O [34]. Later diagenetic dissolution of allochems rich in Mg-calcite (red algae and large benthic foraminifera) in đăng nhập 188bet process of porosity inversion, might be driven by acidification of connate marine waters by CO2 diffusion from đăng nhập 188bet hydrocarbon reservoir down into đăng nhập 188bet aquifer [35]. With đăng nhập 188bet beginning of CO2 and hydrocarbon charging đăng nhập 188bet reservoir, đăng nhập 188bet gas water contact and hence đăng nhập 188bet dissolution of Mg-calcite-rich allochems moved downward, causing đăng nhập 188bet amount of Ca2+, CO32, and also Mg2+ (mainly derived from dissolved corraline red algae) to increase in đăng nhập 188bet underlying aquifer [36]. 

Partial dolomitisation, observed in đăng nhập 188bet Serravallian section of both Well-2 and Well-3, is rare to non-existent in Well-4. Dolomitisation appears to post-date meteoric cement and pre-date deep burial-realm diagenesis. It is challenging to predict đăng nhập 188bet extent of dolomitisation across đăng nhập 188bet platform, given đăng nhập 188bet limited number of well penetrations. Onset of clastic deposition occurred during đăng nhập 188bet Tor1 sequence, constraining đăng nhập 188bet initiation of dolomitisation to Early Tortonian time. Dolomitisation may continue to take place after initiation of burial by shales and marls, as tested in reactive transport modelling [37]. đăng nhập 188bet observed partial dolomitisation at Well-2 and Well-3 might be related to CO2 and hydrocarbon charging, causing secondary leaching of đăng nhập 188bet Mg-rich coraline red algae and LBFs (see above). Enrichment of đăng nhập 188bet water with Mg-ions might have favoured downward dolomitisation, driven by gas displacement, of đăng nhập 188bet Serravallian carbonates [36]. This interpretation is supported by đăng nhập 188bet fact that dolomitised sections of Well-2 and Well-3 are overlain by porous (leached) carbonates.

It is important to note, that only Serravallian carbonates are dolomitised. Dolomitisation does not cross đăng nhập 188bet Serravallian to Langhian boundary (Serravallian sequence boundary Ser1_SB). This argues for an internal and very local Mg source but might also be caused by đăng nhập 188bet different grain composition of đăng nhập 188bet Serravallian compared with đăng nhập 188bet Langhian. đăng nhập 188bet Langhian is dominated by aragonitic bioclasts like corals, whereas đăng nhập 188bet Serravallian is dominated by bioclasts built by Mg-calcite, like coralline red algae. One might argue, that dolomitisation needs high-Mg calcite seeds or nucleation sites (missing in đăng nhập 188bet Langhian) to trigger (“jump-start”) dolomitisation [38, 39].

5. Reservoir rock types (RRT) and geologic controls on reservoir architecture

Reservoir rock typing was done in an effort to model varying reservoir quality, tied to different environments of deposition (EOD) across đăng nhập 188bet Ca Voi Xanh platform. Reservoir rock types (RRT) were identified based on lithofacies, diagenesis, and reservoir parameters. Eight RRTs were identified: six in đăng nhập 188bet Serravallian (RRT-1, RRT-2, RRT-3, RRT-4, RRT-5, and RRT-6; described in detail below) and two in đăng nhập 188bet Langhian (RRT-3L and RRT-5L; Figure 5).

Reservoir rock type 1 (RRT-1)

RRT-1 (Figure 6) islimitedtoWell-2 corematerial(Figure 2). However, it also occurs as pre-dolomitised RRT-1 within đăng nhập 188bet dolomitised part of Well-3 core material (Figure 2). This RRT is characterised by large benthic foraminifera (LBF), encrusting coralline red algae, and, often horizontally aligned, rhodoliths and platy corals. RRT-1 is đăng nhập 188bet only

RRT that shows considerable amounts of mud, giving it a packstone to mud-lean packstone texture (LBF-rhodolith packstone to mud-lean-packstone). Compaction features like horizontal stylolites and solution seams are common.

RRT-1 is interpreted to represent a low-energy, deeper marine, distal middle shelf environment); most likely in an estimated 50 to 100m water depth range.

Throughout đăng nhập 188bet Serravallian section at Well-2 and Well-3 (pre-dolomite RRT-1), intervals representing RRT-1 occur commonly around interpreted maximum flooding surfaces (MFS). This interpretation is further supported by biostratigraphy.

đăng nhập 188bet pore system is dominated by microporosity [40, 41]. Subordinate partial moldic and between particle porosity is partly occluded by blocky calcite cement. RRT- 1 is preferentially dolomitized (see RRT-4).

Reservoir rock type 2 (RRT-2)

RRT-2 (Figure 7) is present in core material of all three wells studied (Figure 2). It is made up of similar allochems characteristic for RRT-1 (but lacks considerable amounts of mud) and RRT-3. RRT-2 is described as rhodolith-LBF grainstone.

đăng nhập 188bet graistone texture indicates elevated to high water- energy conditions. Environment of deposition (EOD) ranges from proximal to mid middle shelf, most probably representing rather shallow water depth of around 30m.

Cementation with circumgranular and blocky cement has considerably reduced between particle, partial moldic, and moldic pore space. Apparent microporosity (Figure 7) is within đăng nhập 188bet grains. RRT-2 represents a strongly cemented version (strong diagenetic overprint) of RRT-3 (see below).

đăng nhập 188bet presence of RRT-2 near interpreted sequence boundaries suggests that cementation most likely is related to exposure. This interpretation is further supported by đăng nhập 188bet presence of cryptocristalline meniscus, circumgranular (but not isopachous), dog tooth, syntaxial, and blocky cements, as well as δ18O and δ13C isotope, and optical fluid inclusions (OFI) analyses; all indicating that cementation took place early in đăng nhập 188bet meteoric phreatic diagenetic environment (Figure 4).

Reservoir rock type 3 (RRT-3)

RRT-3 (Figure 8) dominates đăng nhập 188bet Seravallian section at Well-1, but it is also present, although to a lesser degree, at Well-3 and Well-4 (Figure 2). Similar to RRT-2, RRT-3 is described as rhodolith-LBF grainstone to mud-lean packstone.

Like RRT-2, RRT-3 is characterised by LBF (Cycloclypeus, Katacycloclypeus, Amphistegina, Lepidocyclina) and rhodoliths, interpretedtohavebeendepositedthroughout đăng nhập 188bet proximal to distal middle shelf under moderate to high water-energy conditions. An absolute water depth is difficult to assign to this type of rhodolith-dominated carbonates. However, LBF assemblages, đăng nhập 188bet occurrence of smaller benthic foraminifera, and rhodolith morphologies suggest that it may vary between 30 to 80m.

đăng nhập 188bet pore system is dominated by between particle, partial moldic, and moldic porosity. Cement is present

(cryptocrystalline meniscus, circumgranular, isopachous, syntaxial, and blocky), but it does not occlude all pore space. Cement types represent marine vadose (cryptocrystallinemeniscuscement), phreatic(isopachous, neomorphosed acicular aragonite, and bladed and high- Mg calcite cements), meteoric vadose (cryptocrystalline meniscus cement), phreatic (circumgranular, dog tooth and blocky, as well as syntaxial cements), and/or burial (syntaxial and blocky to drusy mosaic cements) diagenetic environments.

RRT-3 represents a depositional, minor diagenetically altered, reservoir rock type.

đăng nhập 188bet presence or absence of đăng nhập 188bet above-mentioned cement types is, however, responsible for đăng nhập 188bet wide range in porosity and permeability.

Reservoir rock type 4 (RRT-4)

RRT-4 (Figure 9) is comprised of partially dolomitised carbonates and is limited to đăng nhập 188bet Serravallian (not present at Well-4; Figure 2). RRT-4 is a diagenetic reservoir rock type, overprinting RRT-1, RRT-3, and RRT-5. RRT- 4 is characterised as dolomitised samples with > 12% dolomite. Dolomitisation generally enhances reservoir quality with đăng nhập 188bet exception of samples interpreted to represent dolomitised RRT-5. RRT-5 shows somewhat reduced porosities and permeabilities when dolomitised. However, overall reservoir quality remains very good.

đăng nhập 188bet pore system is dominated by intercrystalline, between particle, partial moldic, and moldic porosity, as well as minor microporosity (Figure 7).

Sediment texture and structure-mimetic dolomitisation is interpreted to occur early in đăng nhập 188bet burial history after early dissolution and phreatic cementation, but prior to stylolite formation and late cementation. This event was likely initiated between 10.5 and 10Ma during sea-level rise that followed platform exposure, causing platform-top flooding and eventual platform demise (drowning).

Numerous models exist, discribing various diagenetic environments where dolomitisation is interpreted to take place [42]. One interpretation is that đăng nhập 188bet freshwater lens thickness, which is relatively easy to determine based on đăng nhập 188bet exposed platform or island dimensions, acts as an insulator to đăng nhập 188bet dolomitisation front. This scenario favours seawater being đăng nhập 188bet most likely dolomitisation fluid. More recently, đăng nhập 188bet hypothesis of dolomitisation from early CO2 charge was proposed by Rivers et al. [36]. Post-burial rapid charge of hydrocarbon and CO2 displaces connate water and causes dissolution of magnesium calcite at đăng nhập 188bet gas- water contact; as described above. Dissolution of red algae (high-Mg calcite) and resulting Mg-ion concentration drives aqueous systems near đăng nhập 188bet gas-water contact toward high-Mg calcite and dolomite saturation. Simulation and reactive transport modeling of CO2 gas injection into carbonate reservoirs shows CO2 can drive dissolution and density driven convective mixing [35]. Dolomitisation ceases abruptly at đăng nhập 188bet Serravallian-Langhian boundary, supporting a local source of magnesium for dolomitisation and/or đăng nhập 188bet interpretations that dolomitisation is triggered by high-Mg calcite (coralline red algae) or dolomite “seeds” (nucleation sites) in đăng nhập 188bet sediment. Rivers et al. [36] concluded that dolomitisation appears to preferentially

affect mud-dominated facies. To test this interpretation, pre-dolomite RRTs were interpreted from core and thin sections of Well-2 and Well-3. đăng nhập 188bet results support đăng nhập 188bet initial conclusions made by Rivers et al. [36].

Reservoir rock type 5 (RRT-5)

Like RRT-3, RRT-5 (Figure 10) is dominantly a rhodolith-LBF grainstone. It dominates Well-3 and Well-4, and, to a minor degree, Well-2 (Figure 2).

RRT-5 can be described as a diagenetically altered (leached) RRT-3. đăng nhập 188bet pore system includes between particle, partial moldic, moldic, as well as minor non-fabric selective vuggy porosity; đăng nhập 188bet latter showing touching vugs. Dominant cement type is meteoric circumgranualar cement. Syntaxial and blocky cements (meteoric and/or shallow burial cements) are less common.

RRT-5 is interpreted to be đăng nhập 188bet result of early meteoric dissolution, most probably related to exposure, and subsequent minor cementation in đăng nhập 188bet meteoric phreatic zone. Later diagenetic, CO2-related dissolution, might have additionally increased porosity.

Reservoir rock type 6 (RRT-6)

Like reservoir rock types RRT-3 and RRT-5, RRT-6 (Figure 11) is described as a rhodolith-LBF grainstone, representing a diagenetically altered (leached) RRT-3. Its vertical distribution is limited to đăng nhập 188bet top of đăng nhập 188bet Serravallian at Well-3 and Well-4 (Figure 2). RRT-6 samples show no cementation, but enhanced leaching.

đăng nhập 188bet pore system is dominated by vuggy (touching vugs), partial moldic, moldic, and between partical porosity. đăng nhập 188bet occurrence of touching vugs and đăng nhập 188bet lack of cementation considerably enhances đăng nhập 188bet permeability of RRT-6, compared to all other reservoir rock types.

Like RRT-5, RRT-6 is interpreted to represent early diagenetic, meteoric leaching of đăng nhập 188bet carbonates, related to exposure (epikarst). However, later diagenetic leaching during burial, related to CO2 invasion is also possible.

6. 3D Seismic Paleo-Structure and Karst Mapping

Paleo-structure reconstructions, showing đăng nhập 188bet antecedent topography of đăng nhập 188bet Ca Voi Xanh platform, can be used to spatially guide đăng nhập 188bet distribution of RRTs. Topographic lows (deeper water) would have a higher probability of being occupied by RRT-1, whereas topographic highs (shallower water) would have a higher probability of being occupied by RRT-3.

Paleo-structure, in conjunction with seismically observed karst can also be used to define potential areas of exposure at top carbonate (Tor1_SB). đăng nhập 188bet extent of exposure is a key control on distribution and probability of meteoric diagenesis associated with RRT-5 and RRT-6.

Two seismic horizons were selected that, when used as a datum, best represent đăng nhập 188bet paleo-structure (antecedent topography) at đăng nhập 188bet onset of Serravallian carbonate depostion and at đăng nhập 188bet time of subsequent exposure at top carbonate (Tor1_SB).

An intra-Langhian seismic horizon (LanN) was used as datum for representing đăng nhập 188bet paleo-structure (paleo-topography) at đăng nhập 188bet onset of Serravalllian carbonate deposition (Figure 12). It is an interpretive smooth surface, based on anchoring sea- level in areas of strong constructional/ aggradational facies on đăng nhập 188bet western and eastern carbonate margins, and interpreting/interpolating paleo-water depth in between these anchor points. đăng nhập 188bet selected surface is one of multiple possible realisations.

Figure 13 shows probability maps of RRT-1 and RRT-3 distribution throughout đăng nhập 188bet lower part of đăng nhập 188bet Serravallian; related to đăng nhập 188bet constructed paleo-structure map. Topographic lows or somewhat deeper water conditions (Figure 13a) correspond to a high probablilty of RRT-1 occurrence (Figure 13b), whereas topographic highs or somewhat shallower water conditions (Figure 13a) correspond to a high probablility of RRT-3 occurrence (Figure 13c).

Within đăng nhập 188bet overburden, an intra- Upper Miocene horizon (TN70) was used to show đăng nhập 188bet inferred extent of exposure at top carbonate (Tortonian paleo-structure; Figure 14). It is a relatively well-defined seismic surface at đăng nhập 188bet top of homogeneous marine shales encountered in all Ca Voi Xanh wells. Below this horizon, đăng nhập 188bet clastic section shows significant thickening due to progradation and loading from đăng nhập 188bet west and southwest.

Figure 15 shows a probability map of RRT-5 and RRT-6 distribution throughout đăng nhập 188bet upper part of đăng nhập 188bet Serravallian; related to đăng nhập 188bet re-constructed Tortonian paleo- structure map. Structural highs (Figure 15a) correspond to more exposure and higher probability of RRT-5 and RRT-6 (Figure 15b). On đăng nhập 188bet contrary, structural lows correspond to less exposure and lower probability of RRT-5 and RRT-6 (Figure 15b).

Observations from đăng nhập 188bet 3D seismic data indicate that there is an extensive

karst network on đăng nhập 188bet Ca Voi Xanh structure (Figure 16). Different karst geometriesandfeaturesconsistentwithknownhydro-geologicprocesses are identified in đăng nhập 188bet seismic discontinuity cube (variance cube) and are used to interpret karst regions (Figure 17). A variance volume draped on đăng nhập 188bet top carbonate structure map illustrates discontinuities in đăng nhập 188bet seismic data, interpreted to be karst-related features (Figure 16).

Observations from seismic data indicate that đăng nhập 188bet platform was not karsted uniformly. There is a high-density network of karst features in đăng nhập 188bet northern part of đăng nhập 188bet platform, but they are more sparsely distributed in đăng nhập 188bet central and western areas. Reduced imaging quality underneath gas-filled siltstones makes it difficult to determine đăng nhập 188bet extent of đăng nhập 188bet karst network around Well-2 and Well-3 in đăng nhập 188bet main field area (Figure 16). Vertical trends in karst density honour đăng nhập 188bet Ser1_SB and Tor1_SB sequence boundaries, resulting in more karst/fractures being populated near these interpreted exposure surfaces (Figure 17). Karst characterisation comprises đăng nhập 188bet non- matrix component of đăng nhập 188bet geologic model [43, 44].

7. Conclusions

Offshore Da Nang (Central Vietnam), Middle Miocene (Langhian and Serravallian) carbonates build đăng nhập 188bet reservoir of đăng nhập 188bet isolated Ca Voi Xanh platform; nucleating on top of đăng nhập 188bet north-south striking Tri Ton horst (Figure 1).

Serravallian carbonates can be described by one third-order

composite sequence, comprising three third-order depositional sequences. Four major, regionally correlative sequence stratigraphic surfaces, can be carried across đăng nhập 188bet Ca Voi Xanh platform (Figure 2).

Langhian carbonates are characterized by a photozoan grain association, dominated by hermatipic corals, large benthic (Miogypsina) and, predominantly, small benthic foraminifera, including miliolids, soritids, and alveolinids. A warm, shallow marine, inner shelf environment of deposition is inferred for đăng nhập 188bet Langhian carbonates.

Serravallian carbonates are interpreted to represent a deeper open marine, proximal to distal middle ramp environment. đăng nhập 188bet carbonates are rich in large benthic foraminifera (LBF). Serravallian carbonates represent a typical heterozoan association of organisms.

Early and later diagenetic events rather than primary depositional facies mainly influence reservoir properties. Early, meteoric diagenesis (both vadose and phreatic) is tied to exposure of đăng nhập 188bet carbonates, due to pronounced sea-level falls at đăng nhập 188bet end of Langhian and Serravallian times (Figures 3 and 4). Cement types (cryptocrystalline meniscus, circumgranular, dog tooth, and blocky cements), fluid inclusion and stable isotopes, following đăng nhập 188bet “inverted J” trend, support đăng nhập 188bet interpretation that đăng nhập 188bet carbonates were overprinted by meteoric diagenesis below sequence boundaries (Figure 4b and 4c).

Later diagenetic dissolution of Serravallian allochems rich in Mg-calcite (coralline red algae, and large benthic foraminifera) in đăng nhập 188bet process of porosity inversion, might be driven by acidification of connate marine waters by CO2 diffusion from đăng nhập 188bet hydrocarbon reservoir down into đăng nhập 188bet aquifer.

Partial dolomitisation of Serravallian carbonates, observed at Well-2 and Well-3, is interpreted to be related to CO2 and hydrocarbon charging, causing secondary leaching of đăng nhập 188bet Mg-rich coralline red algae and LBFs. This leads to đăng nhập 188bet enrichment of đăng nhập 188bet water with Mg- ions, favoring downward dolomitisation of Serravallian carbonates. đăng nhập 188bet fact that dolomitisation does not cross đăng nhập 188bet Serravallian to Langhian sequence boundary Ser1_SB argues for an internal and very local Mg source, but can also be controlled by đăng nhập 188bet different grain composition of đăng nhập 188bet Serravallian (high-Mg calcite dominated mineralogy) compared with đăng nhập 188bet Langhian (aragonite dominated mineralogy). Dolomitisation of Serravallian carbonates is thought to be triggered by high-Mg calcite seeds or nucleation sites, missing in đăng nhập 188bet Langhian.

A reproducible reservoir rock type (RRT) scheme was developed for Langhian and Serravallian carbonates, using a combination of depositional environment, diagenetic overprint, and reservoir parameters (Figure 5). đăng nhập 188bet Langhian is characterised by two RRTs, depending on đăng nhập 188bet degree of cementation (RRT-3L) and dissolution (RRT-5L). đăng nhập 188bet Serravallian RRTs are separated into dominantly packstone (RRT-1; Figure 6) and dominantly grainstone textures (RRT- 2, RRT-3, RRT-5, and RRT-6; Figures 7, 8, 10, and 11). RRTs with grainstone texture show varying degrees of cementation (RRT-2 and RRT-3; Figures 7 and 8) and dissolution (RRT-5 and RRT-6, Figures 10 and 11). Reservoir rock type RRT-4 is a diagenetic RRT, showing partial dolomitisation (>12%; Figure 9). đăng nhập 188bet vertical and lateral distribution of RRTs, supported by seismically-derived paleo-reconstruction of đăng nhập 188bet carbonate platform, adequately describes đăng nhập 188bet matrix component of đăng nhập 188bet Ca Voi Xanh reservoir.

Different karst geometries and features identified in đăng nhập 188bet seismic discontinuity (variance) cube (Figure 16) are used to interpret karst regions with different degrees and/or types of karst. These karst regions are used to populate đăng nhập 188bet non-matrix component of đăng nhập 188bet geologic model. Vertical trendology of karst distribution follows sequence boundaries Ser1_SB and Tor1_SB, populating more karst/fractures near these interpreted exposure surfaces (Figure 17).

Acknowledgements

đăng nhập 188bet authors gratefully acknowledge đăng nhập 188bet management of đăng nhập 188bet Vietnam Oil and Gas Group (PVN), Petrovietnam Exploration Production Corporation, ExxonMobil Exploration & Production Vietnam Ltd., and Esso Australia Pty. Ltd. for permission to publish this paper. 

We thank Han van Gorsal for identifying đăng nhập 188bet large benthic foraminifera (LBF) and Davide Bassi for his input on đăng nhập 188bet paleoenvironmental settings of fossil coralline red algal assemblages.

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