Sedimentological and diagenetic impacts on sublacustrine fan sandy conglomerates reservoir quality: An example of the Paleogene Shahejie Formation (Es4s Member) in the Dongying Depression, Bohai Bay Basin (East China)

Abstract

A section of the northern Yanjia nearshore subaqueous fan sandy conglomerate was investigated to understand high-quality reservoirs' distribution and controlling factors in the retrogradation and progradation sequences. These conglomerates have been demonstrated to have strong heterogeneity, tightly cemented, and feature low porosity and permeability. To provide an inclusive understanding of the formation mechanisms and factors governing the distribution of the high-quality reservoirs, we utilized integrated methods including seismic, cores description, wireline, petrography, scanning electron microscopy, cathodoluminescence, and mercury intrusion capillarity. The results showed that the retrogradation rock types are dominated by arkose and lithic arkose and the progradation by lithic arkose and feldspathic litharenite. The dominant diagenesis events are compaction, carbonate cementation, and dissolution. The mechanical compaction effect is similar in the progradation and retrogradation sequence. The average compaction index of 0.70 and 0.68 for the transgressive system tract and highstand system tract, respectively, revealed that compaction is the dominant mechanism in reducing reservoir quality in the nearshore subaqueous fan.The carbonate cementation is more extensive in the highstand system tract than the transgressive system tract. The braided channel sandy conglomerate from transgressive system tract and highstand system tract have the best reservoir quality and features type I and II mercury intrusion capillarity cures with large pore throat and good physical properties. However, the braided channel reservoir quality from the highstand system tract developed during the progradation phase is relatively better than the braided channel of the transgressive system tract in the retrogradation phase. These microfacies are characterized by a substantial dissolution of rock fragments and feldspar. The main channel and the interchannel deposited during the transgressive and highstand system tract, respectively, represented by type III mercury intrusion capillarity curves with small pore throats, are non-reservoirs. The formation mechanism of the high-quality reservoirs in the highstand system tract is guided by sedimentary facies, compaction resistant ability, dissolution of rock fragment, and feldspar. The genetic mechanism of the high quality in the transgressive system tract is governed by the sedimentary facies, compaction resistant ability, carbonate and feldspar dissolution. This study revealed that the progradation sequence retained the favorable high-quality in the nearshore subaqueous fan. This study proved that, for adequate assessment of reservoirs in the Yanjia area and similar reservoirs elsewhere, reservoir characterization must link sequence stratigraphy, sedimentary facies, and diagenesis to comprehend the high-quality reservoir distribution to help the predrill evaluation of reservoirs.