LAUSR

Abstract The lithology, lithofacies, reservoir properties and shale oil enrichment model of the fine-grained sedimentary system in a lake basin with terrigenous clastics of large depression are studied taking the organic-rich shale in the first member of Cretaceous Qingshankou Formation (shortened as Qing 1 Member) in the Changling Sag, southern Songliao Basin as an example. A comprehensive analysis of mineralogy, thin section, test, log and drilling geologic data shows that lamellar shale with high TOC content of semi-deep lake to deep lake facies has higher hydrocarbon generation potential than the massive mudstone facies with medium TOC content, and has bedding-parallel fractures acting as effective reservoir space under over pressure. The sedimentary environments changing periodically and the undercurrent transport deposits in the outer delta front give rise to laminated shale area. The laminated shale with medium TOC content has higher hydrocarbon generation potential than the laminated shale with low TOC content, and the generated oil migrates a short distance to the sandy laminae to retain and accumulate in situ. Ultra-low permeability massive mudstone facies as the top and bottom seals, good preservation conditions, high pressure coefficient, and lamellar shale facies with high TOC are the conditions for “lamellation type” shale oil enrichment in some sequences and zones. The sequence and zone with laminated shale of medium TOC content in oil window and with micro-migration of expelled hydrocarbon are the condition for the enrichment of “lamination type” shale oil. The tight oil and “lamination type” shale oil are in contiguous distribution.