Accumulation Conditions And Pattern Of Tight Sandstone Oil Reservoirs Of The Third Member Of The Paleogene Shahejie Formation In The Nanpu Sag,bohai Bay Basin,eastern China

The Nanpu Sag is a typical oil-rich sag in the Bohai Bay Basin with conventional hydrocarbon in shallow-middle area,while unconventional petroleum resources gradually become the main field for exploration in recent years.At present,the unconventional hydrocarbon exploration in the Nanpu Sag is in the preliminary stage in which exists some of problems like the uncertainty of the accumulation conditions and pattern of the tight oil system.This study utilized the third member of Shahejie Formation(Es₃)in the Nanpu Sag as an example,analyzes the basic characteristics of the source rocks and sandstone reservoirs of the Es₃.At the same time,an improved hydrocarbon generation potential method is proposed to quantify the hydrocarbon generation and expulsion characteristics of the source rocks in the study area.Meanwhile,the diagenetic sequence and porosity evolution history of sandstone reservoirs were reconstructed,and the main controlling factors of the high-quality tight sandstone reservoirs in the study area were summarized.Finally,combining the source rocks and sandstone reservoirs,the favorable exploration area and accumulation model of the tight petroleum system is defined.The results show that the Es₃ is a set of high-quality source rocks.The maximum hydrocarbon generation and expulsion intensity of the source rocks were:500×10⁴t/km²and 250×10⁴t/km²,respectively;the hydrocarbon generation volume in the study area was13.7×10⁸t,and the expelled hydrocarbon was 6.9×10⁸t.The tight oil resource in the Nanpu Sag is 0.69×10⁸t.The main sedimentary microfacies of the tight sandstone reservoir in Es₃ sandstones is the subaqueous distributary channels.Petrological characteristics analysis reservoir mainly belongs to feldspathic lithic sandstone and lithic feldspathic sandstone.The reservoirs are also belonging to a set of tight reservoirs with low porosity and low permeability(average porosity 10.1%,permeability 1.3md).At present,the reservoir stays in the Mesodiagensis A stage A.The compaction is the main process for porosity deterioration,while cementation has a relatively small effect on the reservoir,while the dissolution process is commonly observed.The main controlling factors for the development of relatively high-quality sandstone reservoirs under the generally tight background can be summarized as follow:strong hydrodynamic sedimentary environment,strong dissolution,and the existence of early petroleum charging and regional overpressure.Based on the results of source rock and reservoir analysis,it is concluded that there are good accumulation conditions for the tight petroleum system in Es₃ of the Nanpu Sag,and the Gaoshangpu Structural Belt is the most favorable exploration area in the future,so more attention should be paid to it.