Burial Dissolution Research Articles

Constraint of volcano-stratigraphic emplacement environment on the reservoir distribution: A case analysis of rhyolitic volcanic strata in the 2nd member of Jurassic Huoshiling Formation in Wangfu fault depression, Songliao Basin, East China

Abstract This paper takes rhyolitic volcanic strata of the 2 nd member of Jurassic Huoshiling Formation in the Wangfu fault depression, Songliao Basin as an example to discuss the environment type of volcnano-stratigraphic emplacement and distribution pattern of reservoirs and to analyze the control of volcanic strata emplacement environment on reservoir formation. Lithofacies analysis techniques can roughly determine the paleo-landform before volcnano-stratigraphic emplacement according to the types of underlying rocks. Horizon flatten techniques can basically determine the paleo-landform after emplacement according to coal bottom. There are four environment types of volcano-stratigraphic emplacement, namely, sub-sag before and after emplacement (type I), sub-sag before emplacement-low salient after emplacement (type II), low salient before and after emplacement (type III), and low salient before emplacement and sub-sag after emplacement (type IV). The drilling reveals that the volcanic strata of type I emplacement environment have the reservoir distribution pattern of excellent in the lower part and poor in the upper part because of the controlling of capturing of volatile matter, quenching of magma and deep burial dissolution. However, the volcanic strata of type III emplacement environment have the reservoir distribution pattern of excellent in the upper part and poor in the lower part because of the controlling of the weathering leaching, capturing of volatile matters and tectonic activity. The lower part of volcanic strata in type I and upper part of volcanic strata in type III are favorable exploration targets.

Scale and distribution of marine carbonate burial dissolution pores

It is gradually accepted that porosity can be created in burial settings via dissolution by organic acid; TSR derived or hydrothermal fluids. The role of deep-buried carbonate reservoirs is becoming more and more important since the degree and difficulty in petroleum exploration of shallow strata are increasing. A profound understanding of the development scale and prediction of the deep-buried carbonate reservoirs is economically crucial. In addition to the formation mechanism, scale and distribution of burial dissolution pores in burial settings are focused on in recent studies. This paper is based on case studies of deep-buried (>4500 m) carbonate reservoirs from the Tarim Basin and Sichuan Basin. Case studies mentioned includes dissolution simulation experiments proposes that an open system is of crucial importance in the development of large-scale burial dissolution pores, the distribution pattern of which is controlled by lithology, pre-existing porosity, and pore throat structures. These findings provided the basis for evaluation and prediction of deep-buried carbonate reservoirs.

Multiple dolomitization and later hydrothermal alteration on the Upper Cambrian-Lower Ordovician carbonates in the northern Tarim Basin, China

Thick Upper Cambrian-Lower Ordovician carbonates were deposited on a shallow marine platform in the northern Tarim Basin, which were extensively dolomitized, particularly for the Upper Cambrian carbonates. The resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites. Based on petrographic textures, matrix dolomites consist of very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), finely to medium crystalline, planar-e(s) dolomite (Md2), and finely to coarsely crystalline, nonplanar-a dolomite (Md3). Minor cement dolomites include finely to medium crystalline, planar-s(e) dolomite (Cd1) and coarsely crystalline, nonplanar saddle dolomite (Cd2), which partially or completely fill dissolution vugs and fractures; these cements postdate matrix dolomites but predate later quartz and calcite infills. Origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography, isotopic geochemistry (O, C and Sr), and fluid inclusion microthermometry. Md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines (e.g., evaporated seawater) in near-surface to very shallow burial settings, whereas Md2 dolomite was formed from connate seawater in association with burial dissolution and localized Mg concentration (or cannibalization) in shallow burial conditions. Md3 dolomite, however, was likely the result of intense recrystallization (or neomorphism) upon previously-formed dolomites (e.g., Md1 or Md2 dolomite) as the host carbonates were deeply buried, and influenced by later hydrothermal fluids. Subsequent cement dolomite and quartz crystals precipitated from higher-temperature, hydrothermal fluids, which were contributed more or less by the extensive Permian large igneous province (LIP) activity in Tarim Basin as evidenced by less radiogenic Sr in the cement and parts of matrix dolomites. This extensive abnormal hydrothermal activity could also have resulted in recrystallization (or neomorphism) on the previous matrix dolomites. Faults/fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths. However, the basin uplift triggered by the Late Hercynian Orogeny from the Late Permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids, resulting in precipitation of later calcites in which lighter C and more radiogenic Sr components demonstrate such a switch of fluid properties. This study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the Permian LIP activity within Tarim Basin and elsewhere.

Deep burial dissolution of Lower Palaeozoic carbonates and the role of compacted released water from Palaeogene strata in the Zhuanghai area, Jiyang Depression, Bohai Bay Basin, NE China

The Zhuanghai area is located in the northern part of the Jiyang Depression, Bohai Bay Basin, which is a superimposed basin. In this basin, there are very thick Cenozoic sediments that unconformably overlie Lower Palaeozoic rocks, which form buried hills (topographic highs). Through core description and microscopic observations, the diagenetic characters, pore types, reservoir quality and the dissolution characteristics of the Lower Palaeozoic strata have been investigated in the study area. Organic and inorganic gas–liquid bi-phase inclusions with bright orange-yellow fluorescence are evidence of the dissolution of the compacted released water enriched in organic matter released from Palaeogene strata into the Lower Palaeozoic strata. The relatively high homogenization temperatures of fluid inclusions can help explain the temperature condition of secondary low-temperature hydrothermal minerals and the effects of dissolution of the compacted released water on the Lower Palaeozoic carbonates and hydrocarbons within the buried hills. The δ13C values of crystalline calcite in the high-angle fractures of Lower Palaeozoic rocks range from −0.08‰ to −3.5‰, well above those of primary carbonates of the Lower Palaeozoic. The very high values of δ13C of crystalline calcite suggest the effects of dissolution of the compacted released water on the Lower Palaeozoic carbonates. The paper analyses the effects of dissolution of the compacted released water and the character of the reservoir space of the Zhuanggu-10 well in the Zhuangxi Buried Hill and suggests that the Ordovician Majiagou Formation underwent significant dissolution because of the introduction of compacted released water and formed various secondary pores acting as a primary reservoir. The compacted released water that corroded the Lower Palaeozoic carbonates comes from the Palaeogene sediments in the adjacent Zhuangxi Sag. It migrated along the boundary fault of the sag to the Zhuangxi Buried Hill, modified the Lower Palaeozoic carbonates and caused the formation of deeply buried dissolution pores. The dissolution mode of the down-dip faulted block seepage as recognized in the Chengbei 305 to 307 wells is proposed to explain the pore distribution patterns of the buried hills in the research area. Copyright © 2015 John Wiley & Sons, Ltd.

Genesis and evolution of Lower Cambrian Longwangmiao Formation reservoirs, Sichuan Basin, SW China

Abstract Based on observation of outcrops, cores and thin sections and analysis of logging data and experiment, the features, main controlling factors, evolution and distribution of the Longwangmiao Formation reservoirs in the Lower Cambrian, Sichuan Basin, are examined carefully and the distribution of favorable reservoirs is predicted. Mostly fracture-pore type, the Longwangmiao Formation reservoirs are dominantly comprised of residual dolarenite, oolitic dolomite and crystal dolomite, with dissolution cavities and dissolution pores as the main storage space, an average porosity of 4.28%, and average reservoir thickness of 36 m. The formation of the reservoirs is controlled by grain shoal facies, parasyngenetic dissolution and parasyngenetic dolomitization. The reservoirs have experienced four evolution stages, the period of pore formation laid the foundation for storage space types and physical conditions of the reservoirs; the reservoir physical properties were improved further in the hypergene karstification period; the porosities were decreased by minerals filling during the hydrothermal period; and the reservoirs became denser in the burial dissolution and asphaltic filling period. Based on the main controlling factors of Longwangmiao reservoirs, the high geomorphology area between Huayingshan fault and Longquanshan fault is predicted as the most favorable reservoir zone. Exploration breakthroughs will possibly be made in Guang'an-Nanchong-Jiange area.

Deep-burial dissolution and brecciation in dolostone reservoirs: Striking similarities between the Devonian Keg River Formation of western Canada and the Ordovician Ellenburger group of west Texas

Deeper burial dissolution of dolomitized fabrics created most of the reservoir quality in several Devonian Keg River oil pools on the Comet Platform in northwestern Alberta, Canada. This process also created reservoir quality in two Ordovician Ellenburger dolostone oil pools on the Eastern Shelf of the Midland Basin in west Texas, and provided an alternative explanation for reservoir development unrelated to the subaerial karstification model. Several observations confirmed deeper burial dissolution of Keg River dolostones. First, dolomites replaced, or cemented, grains previously sutured by pressure solution. Their subsequent dissolution postdated this pressure solution, confirming burial dissolution. Second, late-forming saddle dolomites dissolved. Third, secondary porosity developed along, or cross cut, stylolites, often in zebra dolostone fabrics, or formed along fracture planes that cut stylolites. These burial fractures often intersected secondary pores, implying a causal relationship. Fourth, dolostone breccia clasts contained stylolites rotated at different angles to each other and the horizon, confirming deeper burial brecciation. In the Keg River Formation on Comet Platform, proximity to conjugate reactivated basement faults that developed off of a master wrench fault, the Hay River Shear Zone, controlled deeper burial dissolution and pool entrapment. Hydrothermal fluids, either calcium rich or acidic in composition, were moved along these faults and promoted extensive dissolution of completely dolomitized fabrics. Ellenburger dolostone reservoirs at Suggs and Withers Fields on the Eastern Shelf of Texas also resulted from deeper burial dissolution of completely dolomitized facies. The Keg River dissolution fabrics were replicated in Ellenburger samples, with the exception of zebra dolostones. Ellenburger reservoir development was closely aligned to faults and fractures associated with a master wrench fault, the Ft. Chadbourne Fault. These faults were inferred to have moved hot (possibly hydrothermal?), deeper burial diagenetic fluids that were calcium-rich, or periodically acidic, promoting dolomite dissolution and secondary porosity development.

Differences between the platform-margin shoal reservoirs and the platform-interior shoal reservoirs of the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The carbonate shoal grainstone reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin provide a typical example of gas exploration in shoal carbonate reservoirs. The carbonate shoal reservoirs in Leikoupo Formation are of two types: platform-margin shoals and platform-interior shoals. Based on a comprehensive data analysis, and through analysis and correlation of grain shoal reservoir characteristics, it has been shown that there are significant differences between platform-margin and platform-interior shoal reservoirs in terms of the types of reservoir space, the relationship between porosity and permeability, and the pore throat structures. The pores in the platform-margin shoal reservoirs are mainly relict intergranular pores and intergranular dissolution pores, and their porosity and permeability show a good positive correlation. The throats are thick and the pore throats have a good distribution. Isolated intragranular solution pores and moldic pores are mainly developed in the platform-interior shoal reservoirs. There is a weak positive correlation between porosity and permeability, pore throats are less developed, and the pore throats have poor configurations. Differences in sedimentary environment and diagenetic evolution are the primary causes of the different reservoir characteristics of the two shoal facies. The water body in the platform-margin shoal deposits had a higher energy than that in the platform-interior shoal deposits, and its wave base was deeper that of the platform-interior shoals. The shoal bodies at the platform margin are much thicker than those in the platform interior. The grainstones of platform-margin shoals are much better sorted. The rocks have undergone compaction during most post-depositional stages. The thicker grainstones of the platform-marginal shoals preserved their original intergranular pores better than the lower-thickness grainstones in the platform-interior shoals. This was favorable for formation of intergranular dissolution pores during later burial. In contrast, the grainstones of the platform-interior shoals are less thick and early intergranular cements are well developed, resulting in large-scale loss of the original intergranular pores. However, as a result of selective dissolution by early fresh water in the meteoric diagenetic environment, intragranular solution pores and moldic pores were readily formed. Thus, the platform-margin shoal reservoirs show preservation of the original intergranular pores enhanced by burial dissolution, but the platform-interior shoal reservoirs are mainly the result of early selective dissolution by meteoric fresh water.

Prediction of buried calcite dissolution in the Ordovician carbonate reservoir of the Tahe Oilfield, NW china: Evidence from formation water

The Lower–Middle Ordovician reservoir of the Tahe Oilfield is dominated by limestones with reservoir spaces formed by the generation of dissolution pores, meaning that buried karst formation can be evaluated by studying water–rock reactions between groundwater and calcite. The hydrogeological information preserved in this reservoir indicates that the Ordovician groundwater were high-salinity and high-closure, characteristics that are of significance to water–calcite reactions. Theoretical chemical thermodynamics combined with equilibrium calcite solution ionization allowed us to establish a dissolution–precipitation evaluation model for calcite, with the theoretical activity of Ca2+ in solution (aCaeq2+) controlled by temperature, pressure, [ΣCO2]−[Ca2+] and solution pH, and with the actual activity of Ca2+ in solution (aCa2+) being controlled by the concentration of various ions in solution. Ionization reaction directions are controlled by ΔG values; these values can be calculated using aCaeq2+ and aCa2+. Here, ground water data were collected from 34 wells that intercepted Ordovician sediments within the Tahe Oilfield, and calcite ΔG values were calculated for these wells. These data indicate that the groundwater in this oilfield favours the dissolution of limestone, with limestones in the west and south of the Tahe Oilfield being more susceptible to dissolution, consistent with observations within the oilfield. The methods employed during this study can also be used to quantitatively assess susceptibility to burial dissolution and to support reservoir evaluation.

Onshore to offshore trends in carbonate sequence development, diagenesis and reservoir quality across a land-attached shelf in SE Asia

Although isolated Miocene buildups in SE Asia commonly form prolific hydrocarbon reservoirs, their equivalents on clastic-dominated land-attached shelves remain poorly known and underexplored. Here, onshore to offshore trends in carbonate development and reservoir quality are assessed across the NW Borneo shelf through study of surface outcrops and subsurface wells. A multidisciplinary programme of fieldwork, petrography and geochemical analyses allowed evaluation of spatio-temporal variations in deposition, diagenesis and pore system development, together with an assessment of controlling influences. In addition to field logging and sample collection >200 samples were studied via transmitted light, cathodoluminescent and scanning electron microscopy together with stable isotopic characterisation (O, C and Sr).Carbonates developed as localised low-, and higher-relief buildups, as well as more continuous sheet-like deposits in near-coast to shelf margin positions. Molluscs, corals, larger benthic foraminifera and coralline algae are common constituents. Most samples show evidence for marine micritisation, and just in shelf margin positions isopachous cements. However, burial diagenesis predominates in the form of compaction, neomorphism, fracturing, late leaching and dolomitisation. Near-coastal carbonates commonly contain siliciclastics, as do some shelf margin deposits that interdigitate with, or are covered in siliciclastics. Some early, probable meteoric leaching affected inner shelf deposits prior to pervasive neomorphic to blocky/poikilotopic calcite cement formation. On the basis of δ18O V-PDB values of −4.5 to −7.9‰ equivalent to δ18O V-SMOW values of 0 to −4‰ at 25–40 °C and δ13C V-PDB values of −0.6 to +1.6‰ cementation probably reflects alteration from terrestrial groundwaters in meteoric aquifers derived from the humid landmass of Borneo. Despite this cementation, moderate energy inner- to mid-shelf grainstones from the core of mounded carbonates still retain, or have enhanced porosity (<8%) over their lower energy counterparts (<4% porosity). Retention of primary porosity and/or late burial dissolution (often associated with saddle dolomite formation) enhancing predominantly primary and minor secondary porosity is key to reservoir quality development in outer-shelf deposits. Best porosity (<20–35%) is in high energy grainstones and rudstones from outer-shelf to shelf-margin positions that experienced minimal clastic influx, most commonly from backstepping to aggradational carbonate sequences. Although stable isotopes for shelf margin calcite cements are consistent with precipitation from marine-derived fluids (δ18O V-PDB values of −3.6 to −5.4‰), those for the late dolomites are suggestive of fluids of meteoric origin (δ18O V-PDB values of −5.2 to −7.4‰ equivalent to values of −0.3 to −6.3‰ V-SMOW at 40–60 °C). Critical factors for reservoir quality development in carbonates from siliciclastic-dominated shelves in the equatorial tropics are: (1) development and preservation of primary porosity, (2) cementation associated with meteoric aquifers draining large humid equatorial landmasses, and (3) burial leaching and fluid pathways.

Carbonate porosity creation by mesogenetic dissolution: Reality or illusion?: Discussion

In their recent discussion of the recognition of late burial dissolution of carbonates, Ehrenberg et al. (2012) use our article (Beavington-Penney et al., 2008) as an example of a study that mistakes late burial (mesogenetic) porosity for pores that formed much earlier in the diagenetic sequence. Commenting on our study of reservoir quality variation in Eocene nummulitic limestones, Ehrenberg et al. (2012, p. 229) note that …

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin, southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin. Their occurrence, like other cases worldwide, can be divided into two locations in general, i.e., platform interior and platform margin locations. Their differences of reservoir features and origins, however, have not been investigated comprehensively due to different exploration degrees. This issue is addressed in this paper, to provide basic data and information for the basin’s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide. We compared the features of these two types of reservoirs in detail, including the depositional and diagenetic features, pore types and petrophysical features. Based on the comparison, the origin of the reservoirs was further discussed. It is shown that the reservoirs in platform interior and platform margin locations differ significantly. The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone, which was weakly compacted and intensely cemented and has undergone meteoric dissolution. Pore types include intragranular dissolution and moldic pores, with low porosities ( 9% porosity and >0.1 mD permeability) and pore-throat structures than the interior reservoirs. These differences were influenced by both primary depositional features and secondary diagenesis. For the interior carbonate reservoirs, early meteoric dissolution, weak compaction and strong cementation are important controlling factors. By contrast, the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization, preservation of primary pores and burial dissolution.

Dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic–carbonate sediments, northwestern Qaidam Basin, northwest China

Petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata, with reservoir quality being a key. The formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated. However, the mechanism of mixed siliciclastic–carbonate reservoirs has not been well studied. In this paper, we address this issue using a case study in the northwestern Qaidam Basin, northwest China (N1–E1+2 age, 2500–4500 m). Dissolution pores and fractures dominate the reservoirs. However, only the fractures were the focus of previous studies. Thus, here we investigate dissolution and its impacts on reservoir formation, providing a complementary understanding of the reservoir formation in the basin. Using core observations and examining thin section, it was discovered that dissolution was both random and followed bedding. The dissolved components primarily included calcareous and gypsum minerals, with fingerprints that are characteristic of burial dissolution. Further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation. The fluids passed through faults and fractures. Dissolution pores were an important component of the reservoir, providing approximately 60% of the porosity. In addition, segments of high porosity generally above 5% are associated with dissolution. Based on these observations, a schematic model was established to explain the impacts of dissolution on reservoir formation. Specifically, organic acidic formation fluids enter tectonic fractures, resulting in dissolution as they pass through them. This dissolution enhances the size of the pore space and the reservoir properties of the rocks, eventually developing a fracture-dissolution reservoir.

Geological characteristics and forming conditions of the platform margin large reef-shoal gas province in the Sichuan Basin

Abstract The large gas province of platform margin reefs and shoals of the Permian-Triassic ChangxingFormation-Feixianguan Formation in the Sichuan Basin is studied as an example to describe the forming conditions and geological characteristics of large strip-like gas provinces of platform margin reefs and shoals. The Sichuan Basin experienced cratonic rifting and basement faulting during the Late Permian. Within a widespread regional ramp, a paleogeomorphology pattern of “three highs and three depressions” were formed. The three negative relieves are, respectively, Chengkou-E'xi trough, Kaijiang-Liangping trough, and Nanchong-Mianyang intraplatform depression, and the three topographic highs are Fengjie-Zhenba platform flat, Shizhu-Yilong platform flat, and Suining-Moxi platform flat. In the transitional areas between platform flats and troughs, large-scale platform margin reef-shoal complexes were developed under high energy environment. Diagenetic processes, such as early stage dissolution, seepage-reflux dolomitization and burial dissolution, contributed to porosity improvement in high-quality dolomite reservoirs along platform margins. High mature source rocks and faults developed within the platform margin reef-shoal complexes, forming better hydrocarbon accumulation conditions than those of intraplatform reefs and shoals. Therefore, the distribution of platform margins controlled that of gas accumulation zones. Paleogeography background of large platform, extensive dolomitization and solution in reef-shoal reservoirs, good underlying and lateral adjacent source rocks, strip assemblages of lithological, structural, and litho-structural oil and gas traps are the four basic elements that determine the formation and distribution of large strip-like gas provinces. The configuration of the four elements in the studied area shows promising exploration prospects of large platform margin oil and gas provinces.

Characteristics and significance of the burial dissolution of dolomite reservoirs: Taking the Lower Paleozoic in eastern Tarim Basin as an example

Abstract Based on the characteristics of core, thin-section, physical properties and carbon, oxygen and strontium isotope, the characteristics of Lower Paleozoic dolomite reservoirs, dissolution features and mechanism in eastern Tarim Basin were studied. Massive dissolution pore-cave dolomite is the main reservoir type in eastern Tarim Basin. The dissolution pores and caves are mainly medium to small with dense distribution. The fillings in dissolution pores and cracks commonly show negative δ 18 O, generally far lower than the surrounding rock; slightly negative δ 13 C, partially positive; slightly high 87 Sr/ 86 Sr. Proportion of CH 4 and CO 2 is high in fluid inclusions and some of them contain high proportion of H 2 S, which demonstrated that the organic matter was at the high evolution stage when the inclusions were captured, suggesting deep burial circumstances. Silicification and high homogeneous temperature also indicate the presence of hydrothermal fluids. Dissolution includes organic acid dissolution and hydrothermal water dissolution. Acid fluid mainly includes organic acid, CO 2 and H 2 S, which were mainly generated in the process of evolution of organic matter, with a little H 2 S from sulfate reduction, a little CO 2 from volcanic activity. As the carrier of acid fluid, hydrothermal fluids overcome kinetic obstruction of dissolution and dissolve the deep dolomite. Major faults and associated faults provide channels for hydrothermal fluids, the acid fluid was further dispersed in dolomites through numerous inter-crystal pores caused by dolomitization and micro-cracks caused by compaction to complete dissolution for dolomite reservoirs.

Primary intergranular pores in oolitic shoal reservoir of lower triassic feixianguan formation, Sichuan Basin, Southwest China: Fundamental for reservoir formation and retention diagenesis

The Lower Triassic Feixianguan (飞仙关) Formation oolitic shoal reservoir in the Sichuan (四川) basin (Southwest China) is currently an exploration and research highlight in China. The reservoir is widely believed to be formed mainly by burial dissolution and/or dolomitization on the basis of primary intergranular pores. In this study, through a comprehensive geological study on the whole basin, the dissolution and dolomitization are suggested not to be the fundamental factor of reservoir formation and there thus may be a possible new fundamental mechanism—the preservation of primary intergranular pores, i.e., the retention diagenesis. Based on this, a complex and multi-stage reservoir evolution and formation model is proposed. In the model, the depositional environment is the basis of reservoir initial formation. Subsequently, early compaction and shallow burial cementation result in the primary reservoir differentiation. Then, multi-stage burial dissolution alters and adjusts the reservoir. Because the last stage gaseous hydrocarbons have little diagenetic impact, the reservoir is formed finally. Therefore, this study presents a possible new fundamental mechanism and evolution model for the reservoir formation. The results can be applied in the regional reservoir predication and shaping exploration strategies, and provide reference for the study of shoal reservoirs in other areas.

Reservoir quality variation on an Eocene carbonate ramp, El Garia Formation, offshore Tunisia: Structural control of burial corrosion and dolomitisation

Study of four closely-spaced El Garia Formation reservoirs (offshore Tunisia) has identified diagenetic controls on variation in reservoir quality. The Ashtart, Didon and Zarat Fields have been affected by significant dissolution porosity, which occurred during burial diagenesis, with vuggy pores preferentially forming in micritic sediment. However, inter-crystalline porosity due to dolomitisation is the principal control on reservoir quality in the Hasdrubal Field. This spatial variation in porosity type reflects, to some extent, the structural and sedimentological isolation of Hasdrubal within the facies trend, surrounded by deep water embayments and, unlike the other studied fields, not directly underlain by salt diapirs. Burial corrosion appears to have formed where mid to outer ramp, mud-rich El Garia Formation facies directly overlie salt domes, a consequence of the circulation of aggressive fluids related to increased heat flow above the salt. Inorganic CO 2, focused into the reservoir prior to hydrocarbon emplacement by faults related to salt movement, is the most likely candidate responsible for the burial dissolution porosity observed in the Ashtart, Didon and Zarat fields. A salt diapir does underlie the most basinward (north-east) margin of Hasdrubal, and minor dissolution porosity is present, but we conclude that the absence of significant dissolution in Hasdrubal is due to its offset position, away from the highly faulted roof zone immediately above the salt. Although we favour this model, we cannot rule out that the absence of dissolution porosity in Hasdrubal may reflect the early emplacement of hydrocarbons from an older (non-Bou Dabbous Formation) source, or differences in the timing of salt movement and consequent fault creation below Hasdrubal compared to the other fields, local temperature and/or acid concentration effects, or a different source of CO 2. Differences in the distribution of dissolution porosity in the Ashtart, Didon and Zarat Fields may be related to the presence of intra-reservoir permeability barriers, due to increased nummulithoclastic (= fragmented Nummulites) content. This may also explain the strata-bound nature of the dolomite in Hasdrubal, which, like the dissolution porosity in Ashtart, is overlain by sediments with an increased nummulithoclastic content. Although dolomite is present in variable quantities in all of the studied fields, the significant volume of dolomite in Hasdrubal may reflect a local ‘plumbing’ effect, with dolomitising fluids entering the reservoir via a fault network that does not affect the other fields.

Burial Dissolution of Ordovician Granule Limestone in the Tahe Oilfield of the Tarim Basin, NW China, and Its Geological Significance

Abstract: With a comprehensive study on the petrology, geology and geochemistry of some Ordovician granule limestone samples in the Tahe Oilfield of the Tarim Basin, two stages of burial dissolution were put forward as an in‐source dissolution and out‐source dissolution based on macro‐microcosmic petrology and geochemistry features. The main differences in the two stages are in the origin and moving pass of acid fluids. Geochemical evidence indicates that burial dissolution fluids might be ingredients of organic acids, CO2 and H2S associated with organic matter maturation and hydrocarbon decomposition, and the in‐source fluid came from organic matter in the granule limestone itself, but the out‐source was mainly from other argillaceous carbonate rocks far away. So, the forming of a burial dissolution reservoir resulted from both in‐source and the out‐source dissolutions. The granule limestone firstly formed unattached pinholes under in‐source dissolution in situ, and afterwards suffered wider dissolution with out‐source fluids moving along unconformities, seams, faults and associate fissures. The second stage was much more important, and the mineral composition in the stratum and heat convection of the fluid were also important in forming favorable reservoirs.

Seasonal variations of modern planktonic foraminifera in the South China Sea

Although foraminiferal census and isotope analyses from sediment cores have been widely applied to paleoceanographic reconstructions in the South China Sea (SCS), their modern counterparts, i.e. foraminiferal shells from sediment traps and plankton tows, have been the subjects of few studies in this region. In this investigation, relative abundances of the most popular species used to reconstruct late Quaternary tropical sea-surface conditions have been studied in plankton tows from the SCS in order to offer better constraints on the interpretation of downcore records. In addition to the seasonal differences, the spatial variations in the faunal assemblage are evident from south to north in the SCS basin based on contemporary tows. For example, the species Globigerinella aequilateralis is found to be relatively dominant in the southern (lower latitude) basin but its relative abundance is significantly higher in planktonic tows than in sediments, indicating its proneness to burial dissolution. Pulleniatina obliquiloculata, an index taxon for the Kuroshio Current, is only abundant in the northern part of the SCS basin. The measured oxygen isotope compositions (δ18O) of Globigerinoides ruber, Globigerinoides sacculifer, Neogloboquadrina dutertrei, and P. obliquiloculata all show a good relationship with temperature in the mixed layer (Tmix). The carbon isotope compositions (δ13C) also display a positive distribution pattern relative to Tmix, but with a less significant correlation in winter months than δ18O. Differences of δ18O between shallower (G. sacculifer and G. ruber) and deep dwellers (N. dutertrei and P. obliquiloculata) accurately record the temperature gradient between the surface mixed layer and 100-m water depth, supporting application of this proxy to the reconstruction of upper water-column stratification.

Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic–Cretaceous): keys for recognition and timing

This paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the Middle East. Three microporous reservoirs were selected: the Cenomanian Mishrif Formation in Iraq; the Kimmeridgian–Tithonian Arab D and the Barremian Kharaib Formations, both in Abu Dhabi. Staining, cathodoluminescence and scanning electron microscopy were used for petrographic observations. In the selected reservoirs, three typical micrite microtextures are distinguished: micro-rhombic, compact anhedral, and rounded. The chronology of diagenesis, based on petrographic observations, shows that mineralogical stabilisation, through high magnesian calcite replacement by low magnesian calcite, led to the formation of micro-rhombic micrites. Non-reservoir facies such as seals present compact anhedral micrites. Correlation between these micrites and high aluminium content suggests alumino-silicate minerals (clays) as a catalyst for pressure dissolution, and thus for reservoir degradation through cementation. Micrites from the upper part of oil reservoirs are dissolved during burial diagenesis. Dissolution is responsible for rounded shape and for reduction of crystal size. It also implies pore diameter and pore-throat size enlargement. Comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro-rhombic micrites.

Characteristics and Genetic Analysis of the Deep‐buried Weathered‐crust Karst Hydrocarbon Reservoirs of the Lower Paleozoic Group in the Tarim Basin

Abstract The genetic analysis of the deep‐buried reservoirs of the Lower Paleozoic carbonate rocks in the Tarim basin is a difficult task involving many factors. Firstly, the object of study is carbonate rocks, which have undergone a long term of modification. Secondly, the rocks are deeply buried with depths of 3800–7000 m in the Tarim basin. The primary reservoir properties formed in the deposition have been strongly modified during the deep burial process. Concurrently, the different burial depths in different areas result in diversities of burial temperature, pressure, underground water, hydrochemistry and various physicochemical changes, which further lead to differences in the diagenetic type, diagenetic property, diagenetic degree and their impacts on the reservoir properties. The Lower Paleozoic Cambrian and Ordovician carbonate reservoirs in the Tarim basin can be grouped into four types, i.e., paleo‐weathered‐crust reservoirs, reef reservoirs, buried karst reservoirs and dolomite reservoirs. This paper presents a detailed discussion on the vertical and horizontal distribution characteristics, morphological division, reservoir properties and the efficiency in accumulating hydrocarbons of the paleo‐weathered‐crust type. Furthermore, its genesis is also analyzed. We proposed that the composition of the carbonate rocks, the tectonic movement with associated fractures and fissures, the paleomorphology and paleoclimate, the sea level fluctuation, and the protection of the pores and fissures by the deep burial diagenesis and burial dissolution are the main factors controlling the formation of the paleo‐weathered‐crust reservoirs. We also consider that the petroleum exploration of the Lower Paleozoic carbonate rocks should be focused on the paleo‐weathered‐crust reservoirs.