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The Lower Permian Lucaogou Formation in the Jimsar sag, Junggar Basin is a typical tight-oil reservoir in China. For effective exploration and production, the formation of a high-quality reservoir must be thoroughly studied. In this work, the tight-oil reservoir was examined using a variety of methods, including core and thin-section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The tight-oil reservoirs were primarily deposited in saline lake environments, which are dominated by variable admixture of dolomite, quartz, feldspar, tuff, calcite and pyrite. Nine main lithofacies were identified: (1) siliceous mudstone, (2) dolomitic siliceous mudstone, (3) dolomitic mudstone, (4) intraclast packstone/grainstone, (5) ooid grainstone, (6) bioclast grainstone, (7) dolomitic siltstone, (8) mixed siliclastic and intraclast grainstone and (9) brecciated dolomitic mudstone. The pore types are classified into four categories: primary intergranular, moldic, intercrystalline and fracture pores. The properties of tight-oil reservoirs are quite poor, with low porosity (ave. 7.85%) and permeability (ave. 0.110 mD) and a small pore-throat radius (ave. 0.086 μm). The tight-oil reservoirs are dominated by the aggradation of a repetitive meter-scale sedimentary facies succession that records distinct lacustrine expansions and contractions. These tight carbonates have also undergone significant diagenetic alterations, such as dolomitization, dissolution, neomorphism and fracture created intercrystalline and moldic pores, vug and fractures; chemical and mechanical compaction and carbonate cementation have decreased the reservoir quality. Variations in reservoir quality in the Jimusar sag are due to a combination of lithofacies type, high-frequency cyclic depositional architecture, dissolution intensity, dolomitization and tectonic related deformation. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Jimusar fine-grained rocks. [ABSTRACT FROM AUTHOR]

This paper investigated the porosity controlling factors for tight sandstone reservoir in the Daniudi gas field, Ordos Basin based on an integrated petrographic, petrophysical and geostatistical analyses, and proposed a comprehensive prediction model for reservoir porosity. Compaction was found to be a key factor for causing reservoir densification. The degree of sandstone compaction appears to be affected by grain sizes and sorting. Under normal compaction conditions (e.g., cement content less than 6%, and with no dissolution), the variation in reservoir porosity with burial depth can be well correlated with grain compositions, grain sizes, and sorting. Based on qualitative examination of the controlling factors for reservoir porosities, geostatistics were used to quantify the effects of various geological parameters on reservoir porosities. A statistical model for comprehensive prediction of porosity was then established, on the assumption that the present reservoir porosity directly relates to both normal compaction and diagenesis. This model is easy to use, and has been validated with measured porosity data. The porosity controlling factors and the comprehensive porosity prediction can be used to quantify effects of the main controlling factors and their interaction on reservoir property evolution, and may provide a reference model for log interpretation. [ABSTRACT FROM AUTHOR]

With the recent discovery of new oil fields around the Dongying Depression, Bohai Bay Basin, a better understanding of the origin of its high-quality lacustrine carbonate reservoirs has become important. The lacustrine carbonates of the upper fourth member of the Eocene Shahejie Formation (ES4s) have been examined using a variety of methods, including core and thin section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The δ13C and δ18O values of the Dongying carbonates differ significantly from Eocene marine values, and the 87Sr/86Sr ratios of carbonate minerals are higher than Eocene marine water and the same as the surrounding Palaeozoic carbonate bedrock. Additionally, the distribution of the carbonates is controlled by the surrounding Palaeozoic carbonate bedrock, all of which indicate their lacustrine origin. Six major carbonate lithofacies are distinguished in the southern gentle slope zone. Bioclastic, oolitic and intraclastic grainstones dominate the littoral to sublittoral zones, while mudstones are mainly distributed in the profundal zone. The ES4s interval is composed of a third-order depositional sequence that contains several small-scale, higher frequency depositional cycles. Overall, stacking patterns of high-frequency depositional cycles are consistent with the general facies variation from basin to shore. Shallow-water grainstones contain both primary intergranular and secondary dissolution pores, whereas the dolomitized mudstones in the profundal zone contain intercrystalline pores and fractures. Diagenetic processes that improve reservoir quality include dissolution and dolomitization. Reservoir quality was diminished by burial calcite cementation and compaction. High-quality reservoirs are mostly distributed in dolomitized grainstones and wackestones, which are deposited in the shoal facies of shallow lakes. Variations in reservoir quality in the Dongying Depression are due to a combination of lithofacies type, dissolution intensity, early dolomitization and tectonic-related deformation. Due to the influx of meteoric waters and shallow burial depth, the grainstones in the littoral zone have the best reservoir quality. Integrated studies have helped to understand the reservoir heterogeneity and hydrocarbon potential of the Dongying lacustrine carbonates. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract The Lower Permian Fengcheng Formation in the Mahu Sag of northwestern China is well-known for its alkaline lacustrine deposits. However, previous research is scarce due to this formation's notable burial depth (up to 5000 m) and limited oil and gas exploration. Using XRD, XRF, micro-XRF, and C-O isotope analyses, we determined: (1) the stratigraphic trends in mineral compositions, (2) the cyclic depositional processes, and (3) paleoclimate variations. The petrologic and geochemical data elucidate depositional and diagenetic processes in the mixed siliciclastic, volcaniclastic and chemical strata of the Fengcheng Formation. Sodium carbonate minerals in the studied core are dominated by wegscheiderite (Na 2 CO 3 ·3NaHCO 3) and trona (Na 2 CO 3 ·NaHCO 3 ·2H 2 O). Nahcolite (NaHCO 3) and a Mg-Na carbonate mineral, northupite (Na 2 CO 3 ·MgCO 3 ·NaCl), also occur, although their abundance is obviously lower than that of wegscheiderite and trona. These substances grew as bottom-growth crystals from lakes overlying tuffs. The volumetrically important Ca-Na carbonate mineral, shortite (Na 2 CO 3 ·2CaCO 3), and some northupite precipitated diagenetically from shallow groundwater brines in unlithified, dark-colored siliciclastic sediment and volcanic ash. Shortite mainly precipitated from the Ca-Na-CO 3 -rich shallow groundwater, whereas northupite precipitated from the Mg-Na-CO 3 -Cl-rich shallow groundwater. Sodium carbonates and tuffs, fine-grained tuffaceous rocks, and siliciclastic rocks formed repetitive centimeter- and meter-scale cycles that are shown as dark-colored lithological layers during lake expansion and contraction stages or during volcanic eruptions. The brine composition, which controlled the precipitation of sodium carbonates, was influenced strongly by volcanism and volcanism-related hydrothermal activity. Changes in climate and volcanic eruptions controlled the deposition of alternating beds of sodium carbonates and dark-colored lithologies. Changes in climate caused the expansion and contraction stages in the lakes. During relatively humid climate stages, diluted water entered the basin and resupplied the lake water with calcium and magnesium during flooding events. Sandy to silty beds were deposited by turbidity currents during this stage. During arid climate stages, bedded sodium carbonates grew in the lakes, and their co-occurrence suggests high atmospheric pCO 2 and high temperatures. The ashes that are provided by volcanic eruptions during arid climate stages can also stop the precipitation of sodium carbonates. Highlights • Sodium carbonates successions consist of alternating light-colored sodium carbonates layer and dark-colored tuff/tuffaceous layer. • The sodium carbonate minerals dominated by wegscheiderite and trona. Shortite and northupite occurred as transitional carbonate minerals. • Climates evolutions and periodic volcano eruptions joint to control the formation of depositional cycles in the sodium carbonates successions. [ABSTRACT FROM AUTHOR]