SHALE OIL POTENTIAL OF LACUSTRINE SHALE IN CANGDONG SAG,HUANGHUA DEPRESSION, BOHAI BAY BASIN: IMPLICATIONS FOR GEOCHEMISTRY AND RESERVOIR CHARACTERISTICS

In this study, core description, X-ray diffraction (XRD), Rock-Eval pyrolysis, optical microscope, and scanning electron microscope (SEM) were used in the qualitative analyses of the characteristics of petrography, pore structures, and oil enrichment. On the basis of total organic carbon (TOC), sedimentary structure and mineral composition, eight lithofacies were identified and controlled by Milankovitch Cycle with an obvious sequence.

In the upper unit (Ek₂¹ and Ek₂²), the organic rich massive siliceous mudstone lithofacies and organic rich laminated dolomitic mudstone lithofacies accounted for the majority and showed high hydrocarbon generation potential. These two lithofacies were stable in lateral distribution with total thicknessgreater than 30 m. In the lower unit (Ek₂³), the organic rich laminated siliceous mudstone lithofacies interbedded with organic-contained layered dolomite lithofacies were deposited with continuous thickness greater than 100m, accounting for the majority of this unit.

The oil saturation index (OSI, S₁*100/TOC) of shale from study area was calculated and ranging from 0.18 to 285 mg/g, with an average of 36.99 mg/g. The hydrocarbon-generating simulation experiments proved that potentially producible oil was presented in the study area when the OSI was greater than 70 mg/g (crossover effect). A geochemical log of GX well showed that crossover effect was mainly observed in shale with Ro>0.75 and burial depth greater than 3130m. The OSI was much higher in the interbedded dolomite in Ek₂³ and ranging from 73 to 285 mg/g with an average of 117mg/g. SEM results showed that shale samples were dominated by interparticle pores and dissolution pores, while organic-matter pores were rarely found. The physical properties of the organic-contained layered argillaceous dolomite lithofacies were better than that of other lithofacies. Overall, the reservoir capacity is controlled by lithofacies, maturity, pore structures, and facies configuration. The interbedded unit is favorable for shale oil enrichment and show good hydrocarbon generation potential in the organic-contained layered dolomite lithofacies. On this basis, Ek₂³ shows good potential for shale oil enrichment, and five sweet spot have identified as the favorable target areas for continental shale oil exploration.