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Reservoir quality and diagenesis of the Permian Lucaogou Formation tight carbonates in Jimsar Sag, Junggar Basin, West China

Yongqiang Yang1,2,Longwei Qiu*1,2,Yingchang Cao1,2,Cheng Chen1,Dewen Lei3,Ming Wan3
1. School of Geosciences, China University of Petroleum, Shandong Qingdao 266580, China;2. Laboratory for Marine Mineral Resources, National Laboratory for Marine Science and Technology, Qingdao,266071, China;3. Research Institute of Exploration and Development, Xinjiang Oilfield Company, Karamay 834000, China


The Lower Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin is the 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 (average of 7.85%) and permeability (average of 0.110 mD) and a small pore-throat radius (average of 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.




Keywords:Lithofacie, Porosity, Diagenesis, Tight carbonate, Junggar Basin

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