ABSTRACT

A large number of small-scale lacustrine sedimentary basins are widely distributed across China. Studies of such basins have been limited. These basins have complex characteristics and thus exhibit significant differences in terms of their hydrocarbon potential. At present, among the known 348 small-scale basins with areas less than 20,000 km2 (<7700 mi2), 13 commercial petroliferous lacustrine basins have been identified. Of these, some are referred to as “small but enriched” because they have hydrocarbon abundances per unit area that are far higher than large- to medium-sized petroliferous basins. Small-scale petroliferous basins can be divided into the following two types based on their characteristics and causes for their small size: remnant and proto–small-scale basins. Remnant basins are sedimentary basins retained from predecessor large basins that were far larger than 20,000 km2 (7700 mi2) and have undergone later modification because of tectonic deformation and erosion; it is conspicuous that later modifications caused their small size. Examples of remnant basins include the Jiuxi, Jiudong, Yanqi, and Santanghu Basins. Proto–small-scale basins are small basins during their entire evolutionary history, and either they did not experience later modifications or the old basin was a small-scale basin before modification and it was their dynamics that caused their small size. According to differences in their formation dynamics responsible for their small size, the proto–small-scale basin can be divided into two subtypes: thermal basins and strike-slip basins. The thermal basin formation and evolution are reflective of a deep thermal origin; that is, there is direct or indirect evidence for existing asthenospheric upwelling that led to basin formation, and examples of thermal basins include the Nanxiang and Jinggu Basins. Strike-slip basin formation was closely related to activity on large strike-slip fault systems, and examples of strike-slip basins include the Yitong, Baise, Sanshui, Baoshan, Luliang, Qujing, and Lunpola Basins.

For these small-scale lacustrine basins, the most important fact contributing to the formation of hydrocarbons and reservoirs is that these basins allowed for the deposition, preservation, and maturation of high-quality hydrocarbon source rocks. Furthermore, three common key factors that significantly affected the hydrocarbon occurrence within small-scale sedimentary basins are as follows: (1) a later modification process that benefits the preservation and maturation of the high-quality source rocks (i.e., the uplift and erosion without the destruction of main source rocks followed by basin subsidence), (2) a high geothermal background characterized by high geothermal gradient and hydrothermal activity, and (3) an elevated deep-lake sedimentation rate (>200 m/m.y. [>656 ft/m.y.]) during deposition of the source rocks within underfilled and balanced-filled lakes.

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