--> --> Carbonate Depositional Attributes and Controlling Factors of Reservoir Property in Saline Lacustrine Basin: A Case Study of Paleogene in Western Yingxiong Ridge, Qaidam Basin (NW China)

2018 AAPG International Conference and Exhibition

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Carbonate Depositional Attributes and Controlling Factors of Reservoir Property in Saline Lacustrine Basin: A Case Study of Paleogene in Western Yingxiong Ridge, Qaidam Basin (NW China)

Abstract

The Qaidam Basin,located in the northern part of the Qinghai-Tibet Plateau, is the highest-altitude oil-bearing basin in the world. A number of commercial oil reservoirs have been discovered recently in the lacustrine carbonates of Lower Ganchaigou Formation (Paleogene) of Western Yingxiong Ridge , suggesting great exploration prospect in this area. Available data from drilling cores, however, indicate that reservoirs are thin bedded and are complex in composition and textures, leading to high heterogeneity. Understandings on the depositional attributes and controlling factors for reservoir property of these lacustrine carbonates will, therefore, give insights into reservoir distribution and exploration targets. This study, based on core investigation, well log and seismic data, XRD, elemental and stable isotopic geochemistry, and computed tomography analysis, gets several key findings on this issue, including: (i) carbonates in the study area were deposited in a saline lacustrine basin, which experienced 4 evolution stages including mesosaline, saline, salt and fresh lake. Clastic, carbonate, evaporite and mixed sediments were deposited in a configuration consisting of depressions and paleohighs, of which carbonates were mainly developed in paleohighs during mesosaline and saline lake stages. These carbonates, influenced by lake salinity, water depth and terrigenous sediment supply, show massive, laminated, patchy and peloidal textures , forming different vertical stack associations and horizontal distribution distribution patterns; (ii) Reservoir space in these lacustrine carbonates includes matrix pores and fractures, the former can be divided into nano-scale dolomite intercrystalline pores, microscale dissolution pores and millimeter-scale inter-breccia pores, the latter can be divided into shear fractures, structural breccia-related chicken wire fractures and sedimentary-related inter-bedded factures; and (iii) Reservoir property was controlled by favorable rock fabric, dissolution of gypsum cements by freshwater and tectonic compression during the late Himalayan orogeny.