Application of astronomical cycle theory to sedimentary evolution and sweet spot prediction of shale oil-gas reservoir
As shale oil-gas exploration and development advances rapidly,astronomical cycle theory has attracted much attention in the prediction of shale oil-gas reservoirs.Deep-water shale is extremely insensitive to well-logging/mud-logging and seismic response,so it restricts the application of traditional methods in shale lithofacies evolution and sweet spot prediction,thus severely restricting the research on stratigraphic correlation and evolution regularity of deep-water shale.The research on the high-frequency evolution regu-larity of deep water shale and the prediction of shale oil-gas sweet spots were carried out from the perspective of astronomical cycle,which can provide a new viewpoint for solving above-mentioned difficulties in hydrocarbon exploration.Based on the climate changes on the scale of ten thousand to hundreds of thousands of years at the earth surface controlled by astronomical cycle,the paper analy-zes the sedimentary evolution mechanism driving the development of shale sweet spots within the thickness range of several meters to tens of meters.Moreover,the three application cases of astronomical cycle theory in prediction of shale oil-gas reservoirs are elabora-ted as below.(1)Based on deep machine learning,the sedimentary autogenetic cycle was accurately identified and eliminated,the pa-per establishes an astronomical cycle framework with the accuracy of about 1 m in the Eocene deep-water shale area of Dongying sag,and it is proposed that the precessional cycle controls the lithofacies cycle of meter-level organic-rich carbonate-bearing/carbonate-rich shale,and the peak stage of long eccentricity corresponds to the favorable sweet spot interval.(2)Astronomical cycle controls the evolution of the sand-mud reservoir in the Middle Eocene sand-interbedded shale oil reservoirs of Zhanhua sag.The long,short ec-centricty and precessional cycle co-control the stage and frequency of interbedded sand,mineral composition of shale and cyclicity of organic content.The research results can provide new ideas for the prediction of sand-interbedded shale sweet spot intervals.(3)An integrated stratigraphic framework of sequence-astronomical cycle in the transition zone of coarse to fine grains and the shale deposi-tional zone of the lake basin was established,and it is proposed that ordered changes in the lithofacies type and scale of coarse and fine grains are subject to the systematic domain level,and the astronomical cycle controls the lithofacies cycles of several meters to tens of meters within the domain.The research results can help solve the difficulty in prediction of coarse and fine sane rocks encoun-tered when drilling horizontal wells,thus providing support for the integrated exploration and development of conventional and un-conventional reservoirs.With the further and continuous exploration and development of shale oil and gas,the distribution of shale oil sweet spots is analyzed from a new perspective of astronomical cycle,which will help improve the optimization accuracy of sweet spot intervals before horizontal well drilling.
astronomical cycleshale oil and gasreservoirhigh-frequency evolutionprediction
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