APPLICATION TO ATECTONIC RESERVOIR DESCRIPTION OF NONMARINE SEQUENCE-STRATIGRAPHIC CONCEPTS IN THE QINGSHANKOU FORMATION, SONGLIAO BASIN WESTERN SLOPE, NORTHEASTERN CHINA

Nonmarine sequence-stratigraphic concepts were applied in a detailed analysis of the Qingshankou Formation based on extensive core studies, cross-sections, 3D seismic and well-logging in the Songliao Basin Western Slope. These provide a chronostratigraphic framework of significant stratal surfaces for predicting lithology and reservoir facies, and proved to be a useful tool to develop a reservoir description for this complex nonmarine formation.

The Songliao Basin, distributed roughly along NNE direction, is a large scale Mesozoic-Cenozoic nonmarine depression basin developed on the basis of faulted basin, and contains the largest oilfield in China, with hydrocarbon resources over 13.6 billion tons. The basin experienced three major tectonic episodes that differentiated its sediments into rift, depression and final structural inversion sequences. The depression-related sequence in the Western Slope consists of the Lower Cretaceous Quantou Formation (K₁q) overlain by the Upper Cretaceous Qingshankou (K₂qn), Yaojia (K₂y) and Nenjiang Formations (K₂n). The second and third members of K₂qn formed by the first large-scale lake-level drop are one of the most significant reservoir-forming assemblages.

Five main depositional facies were recognized in the formation. The primary reservoir facies are the multistory channel sandstones ranging in thickness from 8 to 20 m, which were deposited by delta front distributary channels during base-level drops. The other reservoir facies are turbidite fan sandstones deposited blow the slope breaks. These upward-fining units ranging in thickness from 2 to 5 m with sharp upper and lower contacts were composed of siltstones and muddy siltstones. Deposition of this reservoir occurred during the initial base-level rise in the late lowstand.

Finer-grained deposits sharply overlie the multistory sandstones. The finer-grained deposits consist of thick mudstones of subaqueous inter-distributary and shore- to shallow-lake origin. Deposits of these finer-grained deposits occurred in the transgressive and highstand parts of the sequences. Ten potential sequence boundaries were identified and correlated in the Qingshankou Formation, and they provide a chronostratigraphic framework for predicting lithology and reservoir facies.