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Immature to Low Mature Source Rocks and Tight Reservoirs of Shale Oil Play in Section Two of the Kongdian Formation, Bohai Bay Basin, China


Shale oil is one of the most important unconventional petroleum resources. Lacustrine immature to low mature organic-rich shale strata (Ro > 0.7%), referred as LILOSS thereafter, are widely distributed in China. However, the shale oil potential of LILOSS in China is far from fully understood. We here present a study on LILOSS source rocks and tight reservoirs of shale oil play in Section Two of the Kongdian Formation in Changdong Sag, Huanghua Depression, Bohai Bay Basin, China. Formed in the sedimentary system of delta – turbidite fan – shallow lacustrine – semi deep and deep lacustrine, the multi-rock type Section Two of the Kongdian Formation is approximately 120 to 600 m thick and is mainly composed of oil-bearing dark mudstones, oil shales interlayered with dolomites, siltstones and fine sandstones. As revealed by drill core and logging data, the complex lithology, heterogeneity and spatial distribution of these oil reservoir rocks are controlled by sedimentary facies. Organic geochemical analyses on shale samples, such as solvent extraction or pyrolysis, gas chromatography and mass spectrometry, indicate that semi deep and deep lacustrine dark mudstones and oil shales are the oil source rocks. These rocks contain Type I or II kerogen and have high total organic carbon (TOC > 3%). Their organic maturity is low with Ro values mostly > 0.7%. Other measured organic values are chloroform bitumen “A” > 0.3% and potential yield (S1+S2) > 25 mg/g. Organic geochemical profiles indicate the present depths of immature to low mature hydrocarbon generation threshold is from 1500 to 3500 m. Micro- and nano-scale structural properties (porosity, pore size, type, etc) and mineralogy are studied using light microscope, electron microbeam techniques such as scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM), and X-ray diffraction (XRD). The micro- and nano-scale pores or fractures are very common, including matrix intercrystal, organic, dissolution, interparticle, and intracrystalline pores and dissolution, filled and interlayer fractures. Widespread and thick high-quality source rocks, high contents of brittle minerals, well-developed micro- and nano-scale pores and fractures, shallow burying, overpressure and weak fault activity of the dense Section Two of the Kongdian Formation provide excellent conditions for enrichment and pooling of shale oil in the thick multi-rock type reservoirs.