Types, distribution and play targets of Lower Cretaceous tight oil in Jiuquan Basin, NW China

Abstract

Based on drilling and laboratory data, the formation conditions of tight oil reservoirs in the Jiuquan basin were comprehensively analyzed and the exploration domains were sorted out. The Jiuquan basin underwent three cycles of lake level fluctuation in early Cretaceous, leaving three sets of high-quality source rocks, the Zhonggou, Xiagou and Chijinbao Formations in the Lower Cretaceous. There are two types of reservoir assemblages, source-reservoir in one type and source below reservoir type, and two types of tight reservoirs, argillaceous dolomite and conglomerate. The “sweet spots” control the enrichment of oil and gas. Argillaceous dolomite tight oil reservoirs have the characteristic of “integrated source-reservoir”, with fractures connecting the matrix micro-pores, pore-fracture type and fracture-pore type “sweet spots” distributed in large scale. The sandy conglomerate tight oil reservoirs were formed by source-reservoir lateral connection, and can be divided into source below reservoir type, source-reservoir side by side type and “sandwich” type. The overlapping areas of the favorable facies belts of fan-delta front and the secondary pore developing belts are the “sweet spot” sites. The favorable areas for seeking conglomerate tight oil are fan-delta front deposits around the Qingxi, Ying'er and Huahai sags, with an exploration area of 550 km2; while the area to seek argillaceous dolomite tight oil is the NW fracture developed belt in Qingxi sag, with an exploration area of 100 km2. Key words: tight oil, tight reservoir, argillaceous dolomite, sandy conglomerate body, Lower Cretaceous, Jiuquan Basin