关键词: 东濮凹陷, 咸化湖盆, 薄互层页岩油, 岩相体, 超压, 甜点

Abstract: Dongpu sag in Bohai Bay Basin is a faulted saline lacustrine basin, where the shale is characterized by “multiple thin layers”, leading to complex shale oil enrichment patterns and significant difficulties in sweet spot identification. Based on core testing data of the Member 3 of Shahejie Formation, combined with exploration practices and retrospective evaluation of old wells, this study investigates the enrichment characteristics of thin interbedded shale oil in the saline lacustrine basin of Dongpu sag. The research results indicate as follows. (1) The shale in the Member 3 of Shahejie Formation of Dongpu sag primarily consists of lithofacies assemblages composed of laminated calcareous shale, siltstone, carbonate rock, and mixed rock, exhibiting “box-type” accumulation characteristics. The laminated carbonate shale acts as both source rock and reservoir, characterized by thin single-layer thickness and “four high” (high total organic carbon content, high oil content, high porosity, and high mobility) attributes. Oil and gas in adjacent siltstones, carbonate rocks, and mixed rocks display micro-migration and near-source accumulation characteristics. (2) Overpressure serves as the external manifestation of thin interbedded shale oil enrichment in saline lacustrine basins, and the degree of overpressure development is positively correlated with hydrocarbon mobility. The development depth of overpressure is basically consistent with the initial threshold of shale oil retained hydrocarbons, and the cause of overpressure is closely related to the massive hydrocarbon generation of source rocks. The overpressure development areas in saline lacustrine basins are significantly influenced by salt rocks, locally forming self-sealing systems. (3) Rapid logging and mud logging identification and sweet spot prediction technologies for the thin-interbedded shale oil in saline lacustrine basins have been developed. The technical features show the large overlapping envelope areas both resistivity versus acoustic time difference curves and natural gamma versus acoustic time difference curves, moderate-to-strong seismic event amplitudes, and vertical lithofacies differentiation via P-wave velocity and elastic modulus. (4) A selection criterion for thin interbedded shale oil in saline lacustrine basins was established. Based on three key factors, i.e., favorable lithofacies unit distribution, overpressure development zones, and thermal maturity (vitrinite reflectance ranging from 0.65% to 1.25%), six sweet spots of shale oil intervals in Dongpu sag were delineated. Exploration practice confirms that the thin interbedded shale oil in the Member 3 of Shahejie Formation of Dongpu sag exhibits a box-type accumulation pattern. Diverse shale assemblages developed in high-frequency oscillatory saline lacustrine basins in Dongpu sag possess exploration potential, with the estimated shale oil resources exceeding 4×10⁸t. The research results provide important theoretical guidance for exploration evaluation and sweet spot prediction of thin interbedded shale oil in faulted saline lacustrine basins.

Key words: Dongpu sag, saline lacustrine basin, thin interbedded shale oil, lithofacies unit, overpressure, sweet spot