High-quality Reservoirs Research Articles

Formation mechanism of high-quality reservoirs of Lower Cretaceous volcaniclastic sedimentary rocks in Songliao Basin

High abundance gas reservoirs were discovered in the volcaniclastic sedimentary rocks of Wangfu faulted depression in Songliao Basin, but the distribution pattern and formation mechanism of this type of high-quality reservoirs were not clear. In the Shahezi Formation of Well Block CS6 in Wangfu faulted depression, the void space type, reservoir property, pore throat and pore diameters were studied by means of the casting thin section, helium intrusion porosimetry, mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) methods, moreover, formation mechanism and main controlling factors of the high-quality reservoirs were discussed. Results showed that lithology of high-quality reservoirs consisted of the volcaniclastic sedimentary agglomerate and the volcaniclastic sedimentary breccia, and exhibited the coarsening-upward characteristics. Furthermore, three types (seven subtypes) of void spaces were identified, among which the intragranular vesicles with primary abundant pores were uniquely developed in the high-quality reservoirs of volcaniclastic sedimentary rocks, indicating this high-quality reservoir belonged to the large-pore and small-throat reservoir; porosity and permeability of volcaniclastic sedimentary agglomerate were slightly higher than those of volcaniclastic sedimentary breccia, and both were significantly higher than those of sedimentary rock in the Shahezi Formation, the porosity was closely related with the burial depth. Formation mechanisms of reservoirs in the volcaniclastic sedimentary rocks were release of volatile matters in parent rocks, grain support, dissolution and tectonic modification, among which the former two were the main controlling factors. The high-quality reservoirs over the study area were mainly distributed in the central part of the fan of volcaniclastic sedimentary rocks, among which the grain-supported, coarse-grained volcaniclastic sedimentary rocks with high content of intragranular vesicular detritus and burial depth less than 3000 m were the most favorable area for development of the high-quality reservoirs.

Seismic prediction on the favorable efficient development areas of the Longwangmiao Fm gas reservoir in the Gaoshiti–Moxi area, Sichuan Basin

The Lower Cambrian Longwangmiao Fm gas reservoir in the Gaoshiti–Moxi area, the Sichuan Basin, is a super giant monoblock marine carbonate gas reservoir with its single size being the largest in China. The key to the realization of high and stable production gas wells in this gas reservoir is to identify accurately high-permeability zones where there are dissolved pores or dissolved pores are superimposed with fractures. However, high quality dolomite reservoirs are characterized by large burial depth and strong heterogeneity, so reservoir prediction is of difficult. In this paper, related seismic researches were carried out and supporting technologies were developed as follows. First, a geologic model was built after an analysis of the existing data and forward modeling was carried out to establish a reservoir seismic response model. Second, by virtue of well-oriented amplitude processing technology, spherical diffusion compensation factor was obtained based on VSP well logging data and the true amplitude of seismic data was recovered. Third, the resolution of deep seismic data was improved by using the well-oriented high-resolution frequency-expanding technology and prestack time migration data of high quality was acquired. And fourth, multiple shoal facies reservoirs were traced by using the global automatic seismic interpretation technology which is based on stratigraphic model, multiple reservoirs which are laterally continuous and vertically superimposed could be predicted, and the areal distribution of high quality reservoirs could be described accurately and efficiently. By virtue of the supporting technologies, drilling trajectory is positioned accurately, and the deployed development wells all have high yield. These technologies also promote the construction of a modern supergiant gas field of tens of billions of cubic meters.

Continuous wavelet transforms of spectral decomposition analyses for fluvial reservoir characterization of Miano Gas Field, Indus Platform, Pakistan

Fluvial depositional systems constitute diverse host for the hydrocarbons all over the globe. Nevertheless, predicting the hydrocarbon sands within these assorted sand systems is a challenge for the prospect generation. We apply the continuous wavelet transform of spectral decomposition technique to a 3D seismic data of the Miano Gas Field in the Indus Platform, Pakistan. Full spectrum seismic attributes like the amplitude and root mean square (RMS) detect the reservoir sands down to a restricted reservoir compartment. Nonetheless, the continuous wavelet transforms can be advantageous in imaging and enactment of these clastic reservoirs within the whole reservoir zone. Rock physics reveal high-quality reservoir with an average porosity ∼25% together with the discrimination of lithology/fluids, signifying the bright chances for ascertaining the potential stratigraphic reservoirs.

Main factors for large accumulations of natural gas in the marine carbonate strata of the Eastern Sichuan Basin, China

The natural gas accumulation zone, where the marine carbonate rock strata are developed, was formed in the eastern Sichuan Basin under the influence of several main tectonic movements (Caledonian Movement, Indosinian Movement, Yanshanian Movement, and Himalayan Movement). Most natural gas reservoirs exhibit the structural-stratigraphic traps together with multistage accumulation, late-stage adjustment and reformation, et cetera. The natural gas accumulation zone (or so-called gas reservoir groups) is controlled by the following main factors: multi-sourced and multi-formed hydrocarbons for marine source rocks (i.e. Lower Silurian Longmaxi Formation, Lower Permian, Upper Permian Longtan Formation), paleo-uplift, paleoslope, and the hinge belt controlled by the steep dip structures, namely the Lower and Middle Triassic high-quality gypsum. Three sets of high-quality source rocks (i.e. S1l, P1, P2l) account for the abundant hydrocarbon supply for natural gas accumulation in the eastern Sichuan area, especially in the destructed oil reservoir formed earlier. The said destructed oil reservoir not only provides the preservation space for natural gas reservoir that will take place later, but it also provides the hydrocarbon source for thermal cracking of hydrocarbons and thermochemical sulfate reduction (TSR). Although the gas reservoirs in the eastern part of the Sichuan Basin experienced multi-stage adjustment and reformation at later times, the thick and high-quality gypsum as well as the mudstone, as available caprocks, have offered a good preservation condition for the underlying gas reservoirs. The paleohighs (e.g. Luzhou paleohigh and Kaijiang paleohigh), the Permian platform margin slope, and the structurally transformed slope under the function of the steep dip anticline in the eastern Sichuan not only form the high-quality carbonate reservoir, but they also became favorable for oil and gas accumulation. The difference in hydrocarbon generation history of the source rock and multistage accumulation of hydrocarbons in the eastern part of the Sichuan Basin caused the diversity in the location of the gas reservoirs, where the gas reservoirs with low H2S contents are distributed in the eastern Sichuan Basin (e.g. Jiannan gas field), while the gas reservoirs with high H2S content are located in the northeastern Sichuan Basin (e.g. Puguang gas field).

Geologic characteristics of deep water deposits and exploration discoveries in slope zones of fault lake basin: A case study of Paleogene Shahejie Formation in Banqiao-Qibei slope, Qikou sag, Bohai Bay Basin

Abstract Based on seismic, logging, formation testing, core and lab test data, this study analyzed the sequence division, facies features of deep water deposits and modes, development of large-scale gravity flow, reservoir physical properties and their main controlling factors, and proposed a classification standard and prediction method of favorable exploration areas in deep water area of the Bin1 oil layers of the lower sub-member of the first member of Paleogene Shahejie Formation in Banqiao-Qibei slope zone of Qikou sag, Bohai Bay Basin. The Bin1 oil layers can be divided into three fifth-order sequences, each less than 100 m thick; a set of gravity flow deposits were formed under deep water background in the slope zone, which contains sedimentary micro-facies such as main channel, distributary channel, channel margin, inter-channel mudstone, and turbidite sand sheet in areas without channels, and, in space, has inherited and constructive development features of multistages. It is a sedimentary sequence of fan delta – distal subaqueous fan – deep lake, and every distal subaqueous fan formed by gravity flow can be divided into inner-, middle- and outer fans. The cross-facies transported sands which are sourced from higher-sand-content major sands of delta front can form high quality reservoirs with an average porosity of 15.1% and geometric average permeability of 5.1×10−3 μm2. The main channel and distributary channel of distal subaqueous fan are the most favorable exploration zones.

Impact of diagenesis on reservoir quality and heterogeneity of the Upper Triassic Chang 8 tight oil sandstones in the Zhenjing area, Ordos Basin, China

Reservoir quality and heterogeneity are critical risk factors in tight oil exploration. The integrated, analysis of the petrographic characteristics and the types and distribution of diagenetic alterations in the Chang 8 sandstones from the Zhenjing area using core, log, thin-section, SEM, petrophysical and stable isotopic data provides insight into the factors responsible for variations in porosity and permeability in tight sandstones. The results indicate that the Chang 8 sandstones mainly from subaqueous distributary channel facies are mostly moderately well to well sorted fine-grained feldspathic litharenites and lithic arkose. The sandstones have ultra-low permeabilities that are commonly less than 1 mD, a wide range of porosities from 0.3 to 18.1%, and two distinct porosity-permeability trends with a boundary of approximately 10% porosity. These petrophysical features are closely related to the types and distribution of the diagenetic alterations. Compaction is a regional porosity-reducing process that was responsible for a loss of more than half of the original porosity in nearly all of the samples. The wide range of porosity is attributed to variations in calcite cementation and chlorite coatings. The relatively high-porosity reservoirs formed due to preservation of the primary intergranular pores by chlorite coatings rather than burial dissolution; however, the chlorites also obstruct pore throats, which lead to the development of reservoirs with high porosity but low permeability. In contrast, calcite cementation is the dominant factor in the formation of low-porosity, ultra-low-permeability reservoirs by filling both the primary pores and the pore throats in the sandstones. The eogenetic calcites are commonly concentrated in tightly cemented concretions or layers adjacent to sandstone-mudstone contacts, while the mesogenetic calcites were deposited in all of the intervals and led to further heterogeneity. This study can be used as an analogue to understand the variations in the pathways of diagenetic evolution and their impacts on the reservoir quality and heterogeneity of sandstones and is useful for predicting the distribution of potential high-quality reservoirs in similar geological settings.

Characteristics and controlling factors of shale gas reservoir in the Fuling area, Sichuan Basin

Statistical analysis of massive laboratory analytical data shows that for intervals in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Fuling area, Sichuan Basin, the total organic carbon content is 0.55%–5.89% with an average of 2.54%, presenting a gradual increase trend from top to bottom. Based on whole-rock X-ray diffraction data, the content of clay minerals of the shale is 16.6%–62.8% with an average of 40.9%, gradually decreasing from top to bottom. The content of brittle minerals gradually increases from top to bottom, ranging from 37.2% to 83.4% with an average of 59.1%. Porosity is 1.17%–7.98% with an average of 4.61% through the helium injection method. The horizontal permeability is 0.002–335.209 mD with an average of 23.785 mD by the steady-state method. Massive analytical data from the high-pressure mercury injection method show the micropores are well developed in the shale, and mainly provide the primary specific surface area of pores, while the micropores, transitional pores and mesopores primarily contribute to permeability. The pore types were observed using SEM in combination with argon ion polishing technology. From top to bottom of the shale interval, the organic pores increase and the inorganic pores decrease. Gas desorption data show that the total gas content is 0.44–5.19 m3/t with an average of 1.97 m3/t, and gradually increase from top to bottom of the shale interval. The development of marine shale gas reservoir in Well A is controlled by mineral compositions, the development characteristics of organic matter. TOC is not only a major intrinsic factor controlling the lower reservoir interval, but also an important material to provide the reservoir space for the shale gas. In the late diagenetic stage, the clay mineral assemblages changed from montmorillonite to illite, forming new micropores and leading to an increase of the porosity and great influences on the upper reservoir interval. The content of brittle minerals is more than 50%, facilitating formation of fractures, and the permeability performance can be enhanced significantly. In general, the Wufeng Formation and the bottom interval of the Longmaxi Formation are both high-quality reservoirs and primary pay intervals.

Control of tectonic differentiation on the formation of large oil and gas fields in craton basins: A case study of Sinian–Triassic of the Sichuan Basin

Craton basins are the main sites where marine carbonate oil and gas reservoirs develop in China. However, the previous studies scarcely focused on the controlling effects of tectonic differentiation on the formation of large oil and gas fields in the ancient craton basins. In such circumstances, it is difficult to evaluate and select the favorable zones for exploring carbonate oil and gas. In this paper, based on the research results of the Sinian–Triassic prototype basin and lithofacies paleogeography of the Sichuan Basin, the tectonic differentiation pattern of craton basins and its controls on the accumulation elements and the distribution of hydrocarbons were analyzed with reference to the concept that structures control the sedimentation and distribution of oil and gas. The study reveals the results in three aspects. First, the intra-cratonic graben controls the high-quality source rocks and the hydrocarbon generation center, which forms good source-reservoir assemblage with the high-quality reservoir in the platform margin at the flank, showing excellent near-source accumulation conditions. Second, the three types of paleo-uplifts (i.e. differential denudation, syn-sedimentary, and folding) and the deep-large faults developed in the craton are favorable for the formation and distribution of large-scale carbonate reservoirs. Third, the superimposed areas with tectonic differentiation in multiple periods and multiple types are favorable for forming large oil and gas fields. It is concluded that the marine carbonate rocks show a huge oil and gas exploration potential in the Sichuan Basin. Attention should be paid to the new exploration targets of natural gas, such as the Sinian Dengying Fm. in the platform margin at the east flank of Deyang–Anyue graben, the Lower Cambrian Longwangmiao Fm. on the slope of paleo-uplift in central Sichuan Basin, and the Middle Permian Maokou Fm. in central–west Sichuan Basin, where oil and gas accumulation conditions are favorable.

Reservoir characteristics and control factors of Carboniferous volcanic gas reservoirs in the Dixi area of Junggar Basin, China

Field outcrop observation, drilling core description, thin-section analysis, SEM analysis, and geochemistry, indicate that Dixi area of Carboniferous volcanic rock gas reservoir belongs to the volcanic rock oil reservoir of the authigenic gas reservoir. The source rocks make contact with volcanic rock reservoir directly or by fault, and having the characteristics of near source accumulation. The volcanic rock reservoir rocks mainly consist of acidic rhyolite and dacite, intermediate andesite, basic basalt and volcanic breccia: (1) Acidic rhyolite and dacite reservoirs are developed in the middle-lower part of the structure, have suffered strong denudation effect, and the secondary pores have formed in the weathering and tectonic burial stages, but primary pores are not developed within the early diagenesis stage. Average porosity is only at 8%, and the maximum porosity is at 13.5%, with oil and gas accumulation showing poor performance. (2) Intermediate andesite and basic basalt reservoirs are mainly distributed near the crater, which resembles the size of and suggests a volcanic eruption. Primary pores are formed in the early diagenetic stage, secondary pores developed in weathering and erosion transformation stage, and secondary fractures formed in the tectonic burial stage. The average porosity is at 9.2%, and the maximum porosity is at 21.9%: it is of the high-quality reservoir types in Dixi area. (3) The volcanic breccia reservoir has the same diagenetic features with sedimentary rocks, but also has the same mineral composition with volcanic rock; rigid components can keep the primary porosity without being affected by compaction during the burial process. At the same time, the brittleness of volcanic breccia reservoir makes it easily fracture under the stress; internal fracture was developmental. Volcanic breccia developed in the structural high part and suffered a long-term leaching effect. The original pore-fracture combination also made volcanic breccia reservoir more easily leached by fresh water or groundwater, leading to secondary erosion pores. Volcanic rock weathering obviously has control on reservoir properties, and while the thickness of the weathering crust is 200–300 m, the properties of volcanic rock reservoir are the best. This is attributed mainly to the period during and after the volcano eruption, in which tectonism made the brittle volcanic rock develop a large number of fractures and micro cracks. This has led to the increased permeability of volcanic rock reservoir, the weathering and leaching effect of volcanic rock diagenetic late phase (which also formed lots of secondary pores), and greatly improved reservoir conditions. The overlying Permian Wutonggou formation mudstone provided high-quality cap rock for oil and gas accumulation.

Diagenetic Evolution and Formation Mechanisms of High-Quality Reservoirs under Multiple Diagenetic Environmental Constraints: An Example from the Paleogene Beach-Bar Sandstone Reservoirs in the Dongying Depression, Bohai Bay Basin

Diagenetic Evolution and Formation Mechanisms of High-Quality Reservoirs under Multiple Diagenetic Environmental Constraints: An Example from the Paleogene Beach-Bar Sandstone Reservoirs in the Dongying Depression, Bohai Bay Basin

Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic\u2013carbonate sediments, central Bohai Sea, China

The characteristics and formation mechanisms of the mixed siliciclastic–carbonate reservoirs of the Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and scanning electron microscopy observations, X-ray diffractometry, carbon and oxygen stable isotope geochemistry, and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type I and Type II mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and intrafossil pores, feldspar dissolution pores mainly developed in Type II sediments. Type I mixed sediments are characterized by precipitation of early pore-lining dolomite, relatively weak mechanical compaction during deep burial, and the occurrence of abundant oil inclusions in high-quality reservoirs. Microfacies played a critical role in the formation of the mixed reservoirs, and high-quality reservoirs are commonly found in high-energy environments, such as fan delta underwater distributary channels, mouth bars, and submarine uplift beach bars. Abundant intrafossil pores were formed by bioclastic decay, and secondary pores due to feldspar dissolution further enhance reservoir porosity. Mechanical compaction was inhibited by the precipitation of pore-lining dolomite formed during early stage, and oil emplacement has further led to the preservation of good reservoir quality.

The Early Cambrian Mianyang-Changning Intracratonic Sag and Its Control on Petroleum Accumulation in the Sichuan Basin, China

The older and deeper hydrocarbon accumulations receive increasing attention across the world, providing more technical and commercial challenges to hydrocarbon exploration. We present a study of an asymmetrical, N-S striking intracratonic sag which developed across the Sichuan basin, south China, from Late Ediacaran to Early Cambrian times. The Mianyang-Changning intracratonic sag is ~50 km wide, with its steepest part in the basin center. In particular the eastern margin shows its greatest steepness. Five episodes in the evolutions of the sag can be recognized. It begins in the Late Ediacaran with an uplift and erosion correlated to Tongwan movement. Initial extension occurred during the Early Cambrian Maidiping period, when more strata of the Maidiping Formation were deposited across the sag. Subsequently, maximum extension occurred during the Early Cambrian Qiongzhusi period that resulted in 450–1700 m thick Maidiping-Canglangpu Formations being deposited in the sag. Then, the sag disappeared at the Longwangmiao period, as it was infilled by the sediments. The intracratonic sag has significant influence on the development of high-quality reservoirs in the Dengying and Longwangmiao Formations and source-rock of the Niutitang Formation. It thus indicates that a high probability for oil/gas accumulation exists along the intracratonic sag, across the central Sichuan basin.

Characteristics and accumulation mode of large-scale Sinian-Cambrian gas reservoirs in the Gaoshiti-Moxi region, Sichuan Basin

AbstractThe Sinian-Cambrian formations of the Sichuan Basin have favorable hydrocarbon accumulation conditions, but the exploration for large-scale gas fields is quite challenging due to old strata and multiple tectonic movements. Since the Weiyuan Sinian large gas field was found in 1964, the largest monoblock gas field (Anyue Gasfield) was discovered in the Cambrian Longwangmiao Formation of the Moxi region in 2013 with proven gas reserves of 440.1×109m3. Total proven, probable and possible reserves exceed one trillion cubic meters in the Sinian Dengying Formation and the Cambrian Longwangmiao Formation of the Gaoshiti-Moxi region. The natural gas components, light hydrocarbons, reservoir bitumen abundance and other evidences prove that the dry natural gas was mainly derived from oil-cracking, with methane (a content of 82.65%-97.35%), ethane (a content of 0.01%-0.29%), nitrogen (a content of 0.44%-6.13%), helium (a content of 0.01%-0.06%), and hydrogen sulphide (0.62-61.11g/m3). Gas reservoir pressure increases gradually from the Sinian normal pressure (a pressure coefficient of 1.07-1.13) to high pressure (a pressure coefficient of 1.53-1.70) in the Cambrian Longwangmiao Formation. The temperature of the gas reservoir is 137.5-163 °C. Gas reservoir traps are divided into three categories: tectonic type, tectonic-formation type and tectonic-lithologic type. The large-scale enrichment of the Sinian-Cambrian natural gas results from effective configuration of the large stable inherited palaeo-uplift during the Tongwan tectonic movement, wide distribution of ancient source rocks, high-quality reservoirs with vast pore-cavity, crude oil cracking of large palaeo-reservoirs and favorable preservation conditions. According to the palaeo-structure pattern prior to crude oil cracking of the palaeo-reservoirs, and bitumen abundance as well as the distribution characteristics of current gas reservoirs, the accumulation patterns of the cracking gas reservoir can be classified into three types: accumulation type, semi-accumulation and semi-dispersion type, and dispersion type. This understanding will play an important role in guiding the exploration of the Sinian-Cambrian natural gas exploration in the Sichuan Basin.

Characteristics of Dengying Fm sedimentary sequence in the central–western Sichuan Basin and their controlling effect on gas accumulation

A large gas reservoir in the Sinian Dengying Fm was recently discovered in Anyue area, central Sichuan Basin. In order to promote the oil and gas exploration in this area, it is of great significance to sustainably deepen the analysis on its high-quality reservoir distribution and source-reservoir assemblage. In this paper, a series of studies were performed on the sequence frameworks and sedimentary filling evolution characteristics of the Dengying Fm in central–western Sichuan Basin, and their controlling effects on the reservoir development and distribution and source-reservoir assemblage during its sedimentation. It is shown that the Deyang–Anyue paleo-rift is, on the whole, distributed in a NW direction, and is morphologically wide and steep in the north, but narrow and gentle in the south. There are two rift boundaries of the second and fourth members of Dengying Fm (Deng 2 Member and Deng 4 Member, respectively). The rift of Deng 2 Member is distributed in the shape of “U” in Deyang–Neijiang area. Dengying Fm is divided into four third-order sequences (SQ1–SQ4). In SQ3 of Gaoshiti–Moxi area, central Sichuan Basin, a complete sedimentation reflection structure of transgressive overlap and regressive progradation can be identified with early–middle Deng 4 Member platform margin. The paleo-rift was in embryo before the sedimentation of Sinian, so the sedimentary filling and lithofacies distribution of the Sinian were influenced by the inherited activities of pre-Sinian rifts. The favorable distribution zones of high-quality reservoirs are located at the rift margins of Deng 2 and Deng 4 Members that were formed respectively during the SQ2 and SQ4 and the platform margin of early Deng 4 Member that was formed during the SQ3. A laterally connected source-reservoir assemblage occurs at the margins of Deng 2 and Deng 4 Members around the Lower Cambrian source rocks of the paleo-rift. The northwestward extending margin zone along the eastern flank of the paleo-rift shows a good potential for hydrocarbon accumulation.

Synthetic Fractal Modelling of Heterogeneous and Anisotropic Reservoirs for Use in Simulation Studies: Implications on Their Hydrocarbon Recovery Prediction

Optimising production from heterogeneous and anisotropic reservoirs challenges the modern hydrocarbon industry because such reservoirs exhibit extreme inter-well variability making them very hard to model. Reasonable reservoir models can be obtained using modern geostatistical techniques, but all of them rely on significant variability in the reservoir only occurring at a scale at or larger than the inter-well spacing. In this paper we take a different, generic approach. We have developed a method for constructing realistic synthetic heterogeneous and anisotropic reservoirs which can be made to represent the reservoir under test. The main physical properties of these synthetic reservoirs are distributed fractally. The models are fully controlled and reproducible and can be extended to model multiple facies reservoir types. This paper shows how the models can be constructed and how they have been tested. Reservoir simulation results of a number of generated 3-D heterogeneous and anisotropic models show that heterogeneity, in terms of only the geometric distribution of reservoir properties, has a little effect on oil production from high and moderate quality reservoirs. However, if the effect of heterogeneity on capillary pressure is taken into account, the effect becomes striking, where varying the heterogeneity of reservoirs properties can lead to a 70 % change in the predicted oil production rate and a significant early shift of water breakthrough time. Hence, it is the heterogeneity consequences that are really substantial if not taken into account. These are very significant uncertainties for a hydrocarbon company if the heterogeneity of their reservoir is not well defined.

Petrological and geochemical constraints on diagenesis and deep burial dissolution of the Ordovician carbonate reservoirs in the Tazhong area, Tarim Basin, NW China

Deeply buried (4500–7000 m) Ordovician carbonate reservoirs in the Tazhong area, Tarim Basin, NW China show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27.8% in sour dolostones, from which economically important oils, sour gas and condensates are currently being produced. Petrographic features, C, O, Sr isotopes were determined, and fluid inclusions were analyzed on diagenetic calcite, dolomite and barite from Ordovician reservoirs to understand controls on the porosity distribution. Ordovician carbonate reservoirs in the Tazhong area are controlled mainly by initial sedimentary environments and eo-genetic and near-surface diagenetic processes. However, vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction, filling and cementation. In some locations or wells (especially ZG5-ZG7 Oilfield nearby ZG5 Fault), burial diagenesis (e.g. thermochemical sulfate reduction, TSR) probably played an important role in quality improvement towards high-quality reservoirs. C2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures (Ths) from 86 to 113 °C, from 96 to 128 °C and from 128 to 151 °C, respectively. We observed petrographically corroded edges of these high-temperature minerals with oil inclusions, indicating the dissolution must have occurred under deep-burial conditions. The occurrence of TSR within Ordovician carbonate reservoirs is supported by C3 calcite replacement of barite, and the association of sulfur species including pyrite, anhydrite or barite and elemental sulfur with hydrocarbon and 12C-rich (as low as −7.2‰ V-PDB) C3 calcite with elevated Ths (135–153 °C). The TSR may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations. In contrast, no significant burial dissolution occurred in limestone reservoirs and non-TSR dolostone reservoirs. The deeply buried sour dolostone reservoirs may therefore be potential exploration targets in Tarim Basin or elsewhere in the world.

Reservoir formation mechanism analysis and deep high-quality reservoir prediction in Yingcheng Formation in Longfengshan area of Songliao Basin, China

Although commercial gas flow was produced in several wells with recent years' exploration of Longfengshan area in Changling fault sag, the formation mechanism and controlling factors for high-quality reservoirs still remained undefined. Here, the Yingcheng tight gas reservoirs of Longfengshan area are used as an example to characterize high-quality reservoir formation mechanism and distribution rules. Based on the thin section, SEM, X-ray diffraction, computed tomography (CT) scanning, burial history, constant-rate mercury penetration and physical properties testing, formation mechanism and controlling factors for high-quality reservoirs were analyzed. Results show the following characteristics. First, the reservoir is dominated by chlorite and laumontite cements, and compaction is the most important factor to control reservoir physical properties. According to this, the reservoir can be divided into compacted tight sandstones, chlorite-cemented sandstones and laumontite-cemented sandstones. Second, the high-quality reservoirs are formed due to early extensive laumontite precipitation and the later dissolution of laumontite by organic acid. Meanwhile, it is found that the distribution of cementation and dissolution exhibits some regulations in sedimentary facies, and the distribution is mainly effected and controlled by the lake water and charging of fresh water. Besides, the distribution model of various types of sandstones was established. Studies over diagenesis and sedimentary facies reveal that the high-quality laumontite-cemented sandstones exist in the outside subaqueous fan-delta of the deep sag in Longfengshan area. These findings have been validated by recent exploration wells which obtained high industrial gas flow.

Coupled alteration of hydrothermal fluids and thermal sulfate reduction (TSR) in ancient dolomite reservoirs – An example from Sinian Dengying Formation in Sichuan Basin, southern China

To better understand the fluid activities and their roles in the formation of petroleum reservoirs, dolomite was thoroughly studied in the ancient Sinian Dengying Formation in Sichuan Basin, southern China. The high homogenization temperatures (above 190°C) of fluid inclusions, positive Eu anomaly, and O and Sr isotopic values of coarse crystalline pore-filling dolomite (FD) in dissolution pore spaces have suggested that the FDs are of hydrothermal origin. There are also other minerals (pyrite), organic compounds (pyrobitumen) and gas components (H2S, CO2 and CH4) in pore spaces. The sulfur isotope compositions of pyrite and H2S, along with the carbon isotopes of CO2 have shown the involvement of thermochemical sulfate reduction (TSR). Hydrothermal activities during burial diagenesis of the Dengying Formation provided high temperature environment necessary for TSR to proceed. The presence of CO2 and H2S in both processes enhanced fluid acidities and consequently porosity and permeability of the Dengying Formation. Emplacement of petroleum and acidic fluids helped to maintain pre-existing reservoir spaces during deep burial stage. Overall, the coupled hydrothermal activities and TSR provided favorable environments to allow for maintaining high quality reservoirs.

Dolomite origin and its implication for porosity development of the carbonate gas reservoirs in the Upper Permian Changxing Formation of the eastern Sichuan Basin, Southwest China

High-quality carbonate gas reservoirs of the Upper Permian Changxing Formation (P2ch) in the eastern Sichuan Basin are mainly dolostones. Subsurface core samples from the Longhui-Tieshan area were studied to determine the origins of dolomitizing fluids and interpret porosity evolution during dolomitization. Petrographically, two dolomite types were identified, including micritic to fine-sized dolomites (type 1) and fine-to coarse-sized sucrosic dolomites (type 2). The type 1 dolomites, characterized by micritic crystals (<10 μm) and low Mg/Ca order degrees (mean value of 0.557), selectively replace micritic matrices in carbonates and preferentially occur at intervals rich in micrite, indicating their open, near-surface origin. The type 2 dolomites, including four subtypes, i.e., types 2a, 2b, 2c, and 2d, are characterized by coarse crystals (50 μm ∼ >500 μm) and high Mg/Ca order degrees (mean value of 0.787). Low Fe and Mn concentrations (163 ppm and 69 ppm, respectively), high homogenization temperatures (60 °C–200 °C), depleted δ18O values (mean value of −5.14‰), and Sr isotope compositions (mean value of 0.707643), collectively suggest that the dolomitizing fluid of type 2 dolomites was likely derived from buried, imprisoned brine water closely associated with Early Triassic seawater but unrelated to meteoric water. During the first stage of porosity evolution (i.e., stage A), carbonate porosity was slightly decreased by dolomitizing fluids preferentially replacing micritic components and CO32− ions involvement. During the second stage (i.e., stage B), carbonate porosity was slightly increased because molecular replacement occurred under an environment with insufficient CO32− ions. During the advanced stage (i.e., stage C), dolomitization was intensively weakened because of CO32− ions depletion, yet the acidic dissolution and structural movements produced considerable pores and fractures, forming high-quality reservoirs. Type 1 dolomites are volumetrically minor and have no reservoir potential, effective reservoirs are closely associated with type 2 dolomites. A burial dolomitization model of type 2 dolomites is proposed based on the previous analyses. Dolomitizing fluids moved downward through open faults and unconformities into P2ch carbonates. Potentially high-quality reservoirs likely occur at locations close to dolomitizing fluid conduits.

Characteristics and origin of high-quality lacustrine carbonate reservoirs in the gentle slope area of the Qikou Sag, Bohai Bay Basin

AbstractLacustrine carbonate reservoirs in the gentle slope (ramp) area of Qikou Sag are highly heterogeneous. Some researches about characteristics and distribution of these high quality reservoirs are less. In this study, an integrated investigation was conducted on the high-quality lacustrine carbonate reservoirs in the ramp area of Qikou Sag based on data of thin section examination, lithological log response, testing results, and mercury-injection capillary pressure measurement. In the study area, grainstone reservoirs in the lower 1st member of the Shahejie Formation are high-quality reservoirs characterized by high - medium porosity, moderate permeability, low displacement pressure, and relative large pore throat. Lithologically, these reservoirs are mainly composed of bioclastic and oolitic sparite, and reservoir storage space consists of secondary intergranular dissolved pores and fractures. Hydrodynamic conditions generally control distribution of these reservoirs, as limestone grains filled intergranularly with sparry calcite cement are usually formed under strong hydrodynamic conditions. Sparry calcite cemented limestone is subject to late dissolution with abundant soluble substances, which is one of the direct causes for the well developed secondary dissolved pores. Oolitic bank and bioclastic bank are under high-energy hydrodynamic conditions with the best reservoir petrophysical properties. Rims of beach bars are usually in an intermittently-turbulent highly-hydrodynamic environment with medium reservoir petrophysical properties. Lake bays and supratidal zones are under low-energy tranquil hydrodynamic conditions, with poor reservoir petrophysical properties. During late diagenesis, large quantities of organic acid and slightly acidic water were released from surrounding shale and migrated along the top unconformity of the 3rd member of the Shahejie Formation, resulting in development of numerous secondary dissolved pores in carbonate reservoirs, which is the main cause of high-quality reservoir formation in the sag. Additionally, early charge of hydrocarbons restrained occurrence of authigenic minerals and metasomatism of calcite, and thus promoted excellent preservation of dissolved pores.