Characteristics of reservoir genesis and oil-gas accumulation in lacustrine carbonate in Yingxi area of Qaidam basin

To clarify the reservoir characteristics of typical lacustrine carbonate rocks in the Yingxi area of the Qaidam basin, a large number of cores, thin section, imaging logging and geochemistry data were used to analyze the reservoir characteristics and oil-bearing properties of the Ganchaigou formation (E32) in the study area. It is found that there are inter-salt and under-salt petroleum-bearing combinations in the Yingxi area. The reservoir space are basically intercrystalline pores, with some dissolution pores, tectonic-breccia conglomerate pores and fractures. The development of tectonic seams corresponds to the three episodes of the Himalayan tectonic movement. Formed by laminar lithology differential diagenesis, multi-type diagenetic fractures are important storage space and migration channel of oil. At the same time, intercrystalline pores and dissolution pores are major controlling factors dominating hydrocarbon enrichment and high-production. The results show that: 1) The pores of lacustrine carbonate in the Yingxi area are mainly dolomite intercrystalline pores. The crystal size is 1-2 μm, the pore size is mostly distributed in the range of 424-750 nm, and the face rate is 5%-8%. The dolomite has a penecontemporaneous metasomatic formation mechanism. There is a small amount of intercrystalline solution pores of organic acids. The dissolution pore size is generally less than 1 μm and mostly distributed in the range of 540-990 nm. 2) There are also some salt dissolution pores, tectonic-breccia conglomerate pores and fractures in the study area. Salt dissolution of pores in the reservoir include salt bed dissolution and scattered salt particles dissolution, both of which are contributed by fresh water leaching, and mainly distributed in inter-salt reservoirs. Tectonic-breccia conglomerate pores are formed by faulting and mainly distributed in intra-salt thrust fault zone. 3) Structural fractures and diagenetic fractures develop very well in Yingxi area. The structural fractures are mostly small and middle fracture, with the width of 2-3 mm or less; while diagenetic fractures can be subdivided into three subtypes such as interlayer fractures, shrinkage fractures and stylolite fractures, with the width in the micrometer range. Among them, interlayer fractures are important controlling factors for hydrocarbon accumulation in the study area. 4) Intercrystalline pores are generally oil-bearing and form the basis of hydrocarbon accumulation. Intercrystalline solution pores, salt dissolution pores and tectonic brecciated pores also increase reservoir space and enhance the reservoir performance. © 2019, Editorial Board of Journal of CUMT. All right reserved.