Facies classification and reservoir significance of the Cenozoic intermediate and mafic igneous rocks in Liaohe Depression, East China

Abstract

The facies classification and corresponding features of the Cenozoic intermediate and mafic igneous rocks are studied by analyzing drilling cores, cuttings and corresponding thin sections, as well as well-loggings and seismic profiles related to the boreholes in Liaohe Depression of Bohai Bay Basin. Six facies and sixteen sub-facies are classified: volcanic conduit facies (diatreme, crypto-explosive breccia and post-intrusive sub-facies), explosive facies (pyroclastic flow, surge, and fall deposits sub-facies), effusive lava flow facies (compound flow, tabular flow, hyaloclastite sub-facies), extrusive dome facies (outer zone, intermediate zone, and inner zone sub-facies), volcaniclastic facies (resedimented volcaniclastics, epiclast-bearing volcanogenic deposits sub-facies), and intrusive facies (margin, core sub-facies). The characteristics and recognition of these sixteen volcanic sub-facies are described and summarized in detail concerning their primary volcanic textures, structures, lithologic assemblages, genesis and material source, spatial occurrence and distribution. Volcanic sub-facies is the primary controlling factor on volcanic reservoir spaces and their configurations. They constrain the styles and degrees of subsequent tectonic fracture and secondary dissolution of the volcanics, thus determining the porosity, permeability, and efficiency of volcanic reservoirs. Three favorable reservoir zones in the Cenozoic intermediate and mafic igneous rocks in Eastern Sag of Liaohe Depression are compound lava flows, outer zone of extrusive dome, and margin of intrusive sub-facies. They should be taken as the major exploration targets of the volcanic reservoirs. Key words: Liaohe Depression, Eastern Sag, intermediate and mafic igneous rock, lithofacies, volcanic reservoir, rift basin