Oil and gas enrichment patterns and major controlling factors for stable and high production of tight lacustrine carbonate rocks, a case study of Yingxi area in Qaidam Basin, West China

Abstract

In recent years, sustained important discoveries of oil and gas exploration in the E 32 reservoir of Oligocene in the Yingxi area have been achieved, which revealed great exploration potential in Qaidam Basin. Based on core observations and systematic research on micro-petrological and structural characteristics, lithology, hydrocarbons enrichment patterns, and major controlling factors of stable and high production of the Oligocene reservoir in the Yingxi area of Qaidam were studied. The following findings were reported: (1) the lithology of the target layer is mainly lacustrine carbonate rocks, which can be divided into inter-salt and subsalt strata. In addition, many thick layers of halites widely developed in inter-salt strata. While thin layers of gypsum also developed scatteringly in the subsalt strata. (2) Fine-grained lacustrine carbonate rocks (including dark mudstone) were deposited under saline conditions, which not only are relatively the best quality source rock, but also developed a “dessert” dolomite reservoir. This set of strata is characteristic of “authigenic source rock and authigenic reservoir”. The upper salt layers have a strong sealing ability because of their very high capillary breakthrough pressure and plastic characteristics, and joints formed widely in the subsalt strata since stress concentration. The good matching relationship among all of the above factors led to the formation of favorable area for large-scale hydrocarbon accumulation in the subsalt strata. (3) The development of the matrix intercrystalline pores controls the scale of oil and gas reservoirs and stable production. Moreover, the development of fractures controls the enrichment of oil and gas reservoirs and high production. The research results have important guiding significance for further research on subsalt oil and gas distributing regularities, as well as further exploration deployment of E 32 in the Yingxi area.