The Eocene Qianjiang inter-salt shale play is an emerging lacustrine shale oil target in China, with a potentially prospective area of approximately 2,500 sq. kms in the Qianjiang Depression, Jianghan Basin. The reserve potential for the play is significant, with estimates up to 350 million BOE for one of the 193 inter-salt shale sequences in the Qianjiang Depression. The organic-rich black shales of the Qianjiang Formation were deposited in an inland hypersaline lacustrine basin. The Qianjiang Formation shales accumulated in an environment highly conducive to the production, deposition, and preservation of the organic-rich sediments. Based on industry drilling trends and reported test rates, two major core areas have emerged, each with its unique combination of controlling geologic factors. In regions where structural traps were formed, as a result of salt tectonic development, up-dip lateral oil migration along variably deformed shallow shale laminates may be conducive for hybrid shale oil/ conventional oil systems. In thermally mature source kitchens and the deeper Eq4 section, in contrast, continuous oil accumulation in structurally undisturbed areas may produce unconventional drilling targets that are even more attractive. Similar to other shale-gas and shale-oil plays, key geologic and technical factors defining the Qianjiang Formation inter-salt shale oil plays include shale lithotype, thermal maturity, reservoir pressure, play thickness, porosity, permeability, oil in place, the role of natural fracturing, mineralogy, depth, and the structural style. However, the presence of halite and early oil emplacement are unique for the self-contained, inter-salt, petroleum system. The relative small thickness of the target shales in individual inter-salt deposition cycles and the presence of thick halite rocks as their top and bottom seals also present enormous technical challenges for the resolution of target landing issues, and the ability to maintain natural and artificial fractures open, and free from salt recrystallization during production.
Most of the petroleum produced in China is ascribed to non-marine systems (e.g. Fan et al., 1980; Fu et al., 1988). Changes in past climatic conditions resulted in the formation of various types of source rocks, such as lacustrine evaporites, fresh-water mudstones and coal-bearing formations (Liu et al., 1985), all of which are excellent sources of crude oil in China. The lacustrine evaporitic formations in eastern China have been studied for their petroleum generative potential (e.g. Peters et al., 1996). Geochemical parameters of such source rocks from the Jiangling and Dangyang area in the Jianghan Basin define at least four petroleum systems (Peters et al., 1996), and stable carbon isotopic approach at the molecular level has been used to reconstruct the original depositional environment of organic-rich lacustrine source rocks in the Jianghan Basin (Grice et al., 1998).