Geological characteristics and controlling factors of lithologic reservoirs in southwestern Qaidam Basin

Abstract

In recent years, several small and rich lithologic oil and gas reservoirs have been discovered in the saline lacustrine beach bar sand in the Qaidam Basin. In order to expand the exploration scale of lithologic oil and gas reservoirs, find new exploration targets and zones, and increase storage and production of oilfields, based on coring, logging and seismic data, the controlling factors of oil and gas accumulation in lithologic reservoirs are defined, and some exploration succeeding fields of lithologic reservoirs are pointed out. The results show that: The southwestern Qaidam Basin has four favorable conditions for the formation of lithologic reservoirs. The first is the development of three stable ancient slopes, namely, the Qigequan-Hongliuquan, Shaxi-Yuejin, and Zahaquan-Wunan, which are inherited and conducive to the accumulation of oil and gas. The second is the proximity of two hydrocarbon generation centers, Hongshi and Zahaquan, with superior oil source conditions. The third is that the salt lake basin developed large braided river delta, widely distributed shoal-bar sand and algal hill-gray cloud flat deposits, forming two complementary types of reservoirs of clastic rock and carbonate rock. The fourth is the development of a variety of drainage systems, which can form several sets of oil-bearing strata vertically. Controlled by paleo-structural, sedimentary and diagenetic factors, there are four types of lithologic reservoirs in the southwestern Qaidam Basin: updip pinching out, sandstone lens, physical sealing and dolomitization trap. Lithologic reservoirs are controlled by three factors: paleo-structure, high-quality source rock and effective reservoir. Paleo-structure determines the type of lithologic traps and the direction of hydrocarbon migration. The high-quality source rock controls the distribution and scale of lithologic reservoirs and effective reservoir controls the accumulation of lithologic reservoirs. Comprehensive evaluation shows that the braided river delta front sand body is a favorable area for the exploration of lithologic reservoirs in the lower Ganchaigou Formation of the Lower Paleogene, mainly distributed in the basin dip end of Shaxi, Gasi, and Wunan. The dolomitization trap is a favorable area for exploration of lithologic reservoirs in the upper member of lower Ganchaigou Formation of Paleogene, mainly distributed around the Hongshi Sag. The widely distributed beach bar is a favorable area for lithologic oil exploration in the upper Ganchaigou Formation and lower Youshashan Formation of Neogene, mainly distributed in Yingxiongling, Gasi, Zhahaquan, Wunan, and other areas. This understanding supports a series of major breakthroughs in lithologic exploration, such as well Qietan 2 and the new series of Gasi, with important guiding significance for further exploration of lithologic reservoirs in the southwestern Qaidam Basin.

Keywords

• reservoir formation condition
• controlling factor
• lithologic reservoir
• exploration potential
• qaidam basin