DIAGENETIC EVOLUTION RECORDED FROM dang nhap 188bet FRACTURED CARBONATES OF TRANG KENH FORMATION, IN dang nhap 188bet NORTH-EASTERN AREA OF VIETN
Summary
Diagenetic evolution makes a significant contribution to dang nhap 188bet quality of carbonate reservoir. A detailed geological work was undertaken in a quarry outcrop in Hai Phong city, in dang nhap 188bet North-Eastern area of Vietnam as dang nhap 188bet first step in better understanding and gaining an insight into dang nhap 188bet complex diagenetic evolution of carbonate play. This work integrates dang nhap 188bet results of detailed mapping and interpretation of lithologies and fracture orientations, along with detailed petrographic and isotopic analyses. Across dang nhap 188bet units exposed in dang nhap 188bet studied quarry, textures and isotopic signatures remaining in dang nhap 188bet out crop, illustrate dang nhap 188bet diagenetic history of three substages of diagenetic processes (eogenetic, mesogenetic and telogenetic) that has driven both poroperm reduction and enhancement. Only dang nhap 188bet most recent overprint has enhanced dang nhap 188bet poroperm via secondary poroperm features, such as vugs, solution-enlarged fractures (vuggy fractures), channels, and cavernous porosity, generated by a combination of uplift and meteoric mixing dissolution. Significant levels of poroperm in fractured Devonian carbonates in dang nhap 188bet region are predominant by NE-SW fracture sets that are created and enlarged in dang nhap 188bet telogenetic realm. Equivalents to dang nhap 188bet most important telogenetic substage, recorded in outcrop, likely occur in offshore Devonian subsurface carbonate reservoirs.
Key words: Diagenetic substages, stable isotope, poroperm, enhance, Trang Kenh formation.
1. Introduction
For many years, carbonates have been considered a potential reservoir offshore in dang nhap 188bet North-Eastern area of Vietnam. However, dang nhap 188bet geological characterisation of this carbonate reservoir is so far questionable due to its complex reservoir properties. Until now, many wells testing this play type offshore Northern Vietnam have been failures or show low flow rates, even though they were drilled in high karsted carbonate structures (buried hills). So, dang nhap 188bet question that needs to be answered for this play is: why did they fail and how can we target more successfully in this play type? To answer this, an analog of geological carbonate reservoir needs to be developed, as dang nhap 188bet first step in better understanding and gaining an insight into dang nhap 188bet complicated poroperm development of carbonate plays. We need to understand dang nhap 188bet relative timing of dang nhap 188bet various diagenetic processes, fracture characteristics and fluid evolution that occlude or enhance porosity and permeability. Studying such a reservoir analog in outcrop, at a play-relevant scale, is dang nhap 188bet goal of this current study.
2. Methodology
dang nhap 188bet studied outcrop is in an abandoned quarry (20o58'40''N latitude and 106o41'43''E longitude), in Minh Tan commune, Thuy Nguyen district, Hai Phong city (Figure 1). This quarry is located in dang nhap 188bet Western area of Bac Bo gulf and in dang nhap 188bet Trang Kenh formation (D2gv-D3tk), which also includes dang nhap 188bet various islands of Ha Long Bay, and consists of dark grey Devonian limestone containing abundant corals, stromatoporoids and some brachiopods [5]. dang nhap 188bet outcrop in dang nhap 188bet studied quarry is quite fresh and of sufficient quality for dang nhap 188bet thin section, XRD and isotope analysis.
2.1. Field work
In outcrop, dang nhap 188bet following rock-types are present: matrix-supported mudstone, wackestone, packstone and grainstone (based on dang nhap 188bet combination of field textures and thin section analysis). Both black limestone and grey limestone are recognised in field and show a strong reaction with 10% HCl. Speleothems, carbonate soils in caves, open vugs and caverns are obvious as crosscutting exposures in dang nhap 188bet quarry walls (Figure 2b). In dang nhap 188bet field, early marine cement linings to voids and yellow to buff brown rock types were mapped, this was latter shown in thin section and XRD results to be dolostones and dolomitic limestones.
Carbonate in outcrop is observed as dang nhap 188bet massive or thick laminated carbonate with mostly high dip angles of bedding (40o - 65o), that is affected by multi phases of folding following NW-SE axes. dang nhap 188bet black limestone areas are distributed by flank of two small anticlines as in Figures 2b and 2c.
A total of 193 fracture sets were measured, including calcite veins (mineralised fractures that are calcite filled) and open fractures across dang nhap 188bet studied quarry. Open fractures in dang nhap 188bet quarry have more than two trends: a major WNW-ESE trend, and two minor trends of NNE-SSE and NE-SW. dang nhap 188bet calcite vein orientations can be divided into two trends; with a major NNW-SSE trend and a minor NE-SW trend. That could be affected by dang nhap 188bet first activity of dang nhap 188bet Song Hong fault zone resulted from ENE regional compression (80o) during dang nhap 188bet Oligocene-Miocene to create dang nhap 188bet major NNW-SSE open fracture trending, later filled by calcite (calcite veins). dang nhap 188bet second activity, NNW regional compression from 330o -350o during dang nhap 188bet Pliocene-Quaternary, generates dang nhap 188bet major WNW-ESE open fractures following dang nhap 188bet evidences of dang nhap 188bet Song Hong fault zone in dang nhap 188bet Paleozoic carbonate strata of Cat Ba island [6].
2.2. Petrography and XRD
Based on detailed study of dang nhap 188bet various units exposed in dang nhap 188bet quarry, 19 representative samples were collected (Figure 2b). These samples were slabbed in dang nhap 188bet lab and photographed. Once textures were identified, a subset of texture-aware samples was selected for isotope, thin-section and XRD analysis. Observations and measurements made in dang nhap 188bet field, as well as in dang nhap 188bet laboratory, are now presented.
Petrography and XRD were done to better identify dang nhap 188bet mineral variety and textures observed in dang nhap 188bet outcrop. Early marine cement textures such as morphology, distribution patterns or crystals sizes are visible both in dang nhap 188bet field and in thin section (Figure 3a). In dang nhap 188bet middle zones (turquoise arrow, Figure 3b), of cement-filled pores, blocky calcite with pink to red stained colour, higher iron content, indicates later and hotter burial cement phases. Early medium-grained (10 - 100μm) dolomite lacks stain in thin sections (Figure 4).
Black limestone shows dang nhap 188bet lithology is unstained (calcite) with local black or darker zones that could be due to deposition in a marine environment that was enriched in algae and organic material (Figure 5). XRD confirms dang nhap 188bet most abundant mineral is calcite.
Breccia zones (Figures 2b, 2c, and 6) developed by tectonic activities (damage zones formed) or karst collapse, include dang nhap 188bet large calcite crystals infilling dang nhap 188bet interspace between dang nhap 188bet breccia clasts that were sampled and slabbed for thin section, XRD and isotope analysis. Clasts show a variable packstone to grainstone matrix with bryozoans and oolites clearly visible in thin section.
Speleothems, via meteoric water percolating from overlying soil or bedrock surface in crosscutting cavities and fissures, are abundant in dang nhap 188bet quarry face (Figures 2 and 7).
dang nhap 188bet effects of mechanical and burial compaction are clearly visible in thin section. Figure 8a indicates dang nhap 188bet stress-induced deformation in calcite vein (calcite- filled existing fractures) formed in dang nhap 188bet later stage of mesogenesis. Stylolite in Figure 8b is dang nhap 188bet typical evidence of on-going burial stress, that documents localised pressure solution leading to porosity shutdown in most ancient carbonates residing in mesogenetic substage [7]. In dang nhap 188bet later period, vugs and fractures, formed by dang nhap 188bet dissolution and compaction processes, tend to be more obvious, which can have significant roles in improving fractured reservoir quality (Figures 8c and 8d).
2.3. Isotopes
One hundred isotope samples were collected from slabbed rock sample faces which contain these eight categories in order to capture dang nhap 188bet fluid history of dang nhap 188bet various textures and lithologies. Three groups contain eight categories defined based on dang nhap 188bet textures and lithologies seen in a combination of outcrop, thin section and XRD as described in dang nhap 188bet previous section, namely: 1) early marine cement, 2) dolomite, 3) partly dolomite, 4) grey limestone, 5) black limestone, 6) calcite vein, 7) calcite in breccia zone and 8) speleothem. dang nhap 188bet resulting bivariate isotope plot listing dang nhap 188bet relevant δ18O and δ13C values is presented in Figure 9.
- Group 1 retains on dang nhap 188bet basic trend of somewhat decreasing (but positive) δ13C values from 3‰ to 0‰ (only one carbon value in this trend is negative, i.e., less than 0‰), combined with increasingly negative oxygen values ranging from -4‰ to -12‰. This covariant C-O trend is widely accepted as indicating ongoing burial re-equilibration and recrystallisation tied to increasing temperature and burial depths [7, 8].
- Group 2 (blue points, Figure 9) made up of δ13C values in dang nhap 188bet range 0‰ to +3‰ with strongly depleted δ18O values from -12‰ to -16‰. These are dang nhap 188bet most negative (hottest) oxygen values in all dang nhap 188bet collected samples from calcite-vein filled in interspaces between dang nhap 188bet matrix clasts of dang nhap 188bet breccia zone. dang nhap 188bet highly negative oxygen values suggest dang nhap 188bet possibility of hydrothermal fluids flowing though dang nhap 188bet breccia zone. dang nhap 188bet brittle-response tectonics created entry permeability, via brittle fracture to fluids from or in a hot deep part of dang nhap 188bet mesogenetic realm.
This event took place well after matrix poroperm was lost, as evidenced by dang nhap 188bet complete separation in oxygen value clusters between Groups 1 and 2. These hydrothermal fluids could be related to deep burial and Paleozoic orogeny, or to Tertiary- age deformation and uplift related to major Song Hong fault movements and active hydrothermal circulation tied to dang nhap 188bet effects of Himalayan Orogeny.
- Group 3 (Figure 8) shows moderately to strongly depleted δ13C values ranging from -2‰ to
-14‰ and a narrower range of δ18O values from -6‰ to -11‰. This group includes some calcite veins and a set of speleothems that are dang nhap 188bet precipitates of modern meteoric waters.
3. Discussion
There are three diagenetic substages presented in all ancient sedimentary carbonates, namely eogenetic, mesogenetic, and telogenetic. Each of dang nhap 188bet three drives diagenetic substages that can change dang nhap 188bet intensity of cementation, dolomitisation, compaction, replacement, recrystallisation and solution.
Porosity in dang nhap 188bet carbonates of study area evolved via dissolution, cementation and dolomitisation, first driven by both subaerial and subaqueous processes in eogenetic realm (Figures 3 and 4). dang nhap 188bet isotope signature shows these early marine cements and early dolomites re-equilibrated once they entered dang nhap 188bet mesogenetic realm (Group 1 - burial trend). dang nhap 188bet prevalence of radiaxial calcite in dang nhap 188bet current study area implies these Devonian carbonates of Trang Kenh formation also experienced significant eogenetic porosity loss. Based on thin section analysis done in dang nhap 188bet study area, later dolomite and ferroan calcite (Figure 4) indicate dang nhap 188bet effects of deeper burial with hotter mesogenetic fluids. dang nhap 188bet replacement of earlier carbonate by saddle or sutured dolomite and/or calcite recrystallisation during deep burial diagenesis further reduces porosity. Zones of such porosity destruction are indicated by a depletion both δ18O and δ13C isotope signature values (Figure 9). Ultimately, no eogenetic or early mesogenetic porosity and permeability remains in dang nhap 188bet deeper parts of dang nhap 188bet mesogenetic realm as seen in dang nhap 188bet thin section analysis (Figure 3 - Figure 5 and 3, 9, 12, 14 samples location in Figure 2c).
When dang nhap 188bet telogenesis occurs, dang nhap 188bet carbonate reservoir is exhumed to shallower depth and ultimately to exposure under subaerial condition. dang nhap 188bet isotope signature of dang nhap 188bet meteoric mixing trend is present in dang nhap 188bet study are, and recognised by more negative δ13C values (more exposure to carbon from soil gas weathering) and less negative δ18O values (Figure 8; uplift corresponds to temperature decrease).
Speleothems, formed as meteoric waters pass through vadose zone, have a significant influence on secondary porosity. dang nhap 188bet newly developed vuggy and fissure porosity have been observed at both dang nhap 188bet field and thin section scale (Figure 7). This figure illustrates a variety of pore sizes ranging from tiny vugs to caverns, as well as solution enhanced joints and fissures seen in outcrop. At all scales, telogenesis mostly helps create effective porosity in an otherwise tight carbonate, dissolved by meteoric water.
Fractured zones, solution-enlarged open fractures and channels are widespread in outcrop (Figure 10). These fractures were enlarged by dissolution (vuggy fracture) and now play a significant role in poroperm enhancement, as well as efficient conduits. Dissolution and further fracture porosity enhancement, recognised in this outcrop, can also occur in dang nhap 188bet subsurface when rocks and water are out of chemical equilibrium [9].
Both major NNW-SSE calcite vein and major WNW-ESE open fracture orientations in study area likely correspond to strike-slip displacement of Song Hong fault zone (Figure 1). dang nhap 188bet minor NE-SW calcite vein and fracture trending could be generated by compression during strike-slip. As discussed above, dang nhap 188bet meteoric water will enhance dang nhap 188bet poroperm evolution in carbonate reservoirs. Based on isotope signatures, dang nhap 188bet NW-SW trending calcite veins, that were affected by meteoric mixing, are dominant (Figures 9 and 11).
This observation is useful evidence for future drilling orientations designed to maximise open fracture intersections, as NE-SW trending fractures are related to dang nhap 188bet modern meteoric environment. Most nearby offshore wells testing dang nhap 188bet “buried hill” carbonate play have been drilled as vertical wells either on dang nhap 188bet crest of structures, or following zones of dang nhap 188bet major fault (fracture) damage, but none have focused on fissure trends related to preferential directions of telogenetic enhancement.
Therefore, a future “buried-hill” well should be designed to maximise penetrations of dang nhap 188bet better open-fracture orientations as well as to test multiple intersections with meteoric mixing trending fractures, which much more enhance dang nhap 188bet vuggy and fractures, rather than drilling another test of dang nhap 188bet so-far unsuccessful assumption that hydrocarbons in buried hills reside in a leached and porous cap structure. dang nhap 188bet current study shows there is likely little potential for widespread matrix storage in a buried hill play hosted in Devonian carbonates.
Successful exploration for this type of “buried- hill” target requires deviated drilling and dang nhap 188bet ongoing integration of core, log, seismic, and borehole image data in order to define and predict dang nhap 188bet poroperm distribution in such subsurface reservoir. More integrated approaches using dang nhap 188bet seismic attributes to detect dang nhap 188bet major faults zone, FMI to determine dang nhap 188bet fracture orientation in dang nhap 188bet play area, and an isotope analysis of cuttings to evaluate whether a meteoric mixing zone has been intersected, may define future drilling successes, rather than failures, in this as yet under-explored region of Northern Vietnam.
All substages of diagenetic processes affecting poroperm development and relevant to dang nhap 188bet study area are summarised in Table 1.
4. Conclusion
Field study integrated with petrographic and stable isotope determinations clearly defines all three sets of substages (eogenetic, mesogenetic and telogenetic) remaining in dang nhap 188bet outcrop, but only dang nhap 188bet most recent telogenetic overprint has created dang nhap 188bet porosity and permeability. Poroperm has been destroyed by mesogenetic cementation, dolomitisation, replacement, recrystallisation and compaction. However, porosity and permeability are enhanced in zones of telogenesis. secondary poroperm features, such as vugs, solution- enlarged fractures (vuggy fractures) and channels, and cavernous porosity is generated via major dissolution activity driven by a combination of both uplift and meteoric mixing.
Fracture and calcite vein sets are mainly caused by dang nhap 188bet tectonic and hydrothermal activities that correspond to dang nhap 188bet background structural elements and their diagenetic evolution. NE-SW calcite vein signatures are related to meteoric mixing fluids. This oriented subset of telogenetic processes has significantly enhanced poroperm values in open fractures in what is an otherwise tight carbonate matrix system.
This outcrop study is dang nhap 188bet first step to understand Paleozoic fractured carbonate reservoir plays in subsurface, offshoreVietnam. Combining all interpretation of logs, FMI, cutting data and seismic attributes, core, and thin section analysis results will help develop insightful understanding leading to future drilling success.
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