Depositional and diagenetic control on conglomerate reservoirs: An example from the fourth member of shahejie formation in the Lijin Sag, Bohai Bay Basin, east China

Abstract

The lacustrine conglomerate reservoirs of the Lijin sag, Bohai Bay Basin, East China, were studied to understand the roles of depositional and diagenetic events on reservoir quality, using integrated seismic data, well logs, core observations, optical petrography, SEM-EDS, cathodoluminescence, and fluid inclusion microthermometry. The results show a fan delta with well-connected conglomerate bodies deposited during a highstand system tract (HST) and a nearshore subaqueous fan with weakly connected conglomerate bodies deposited in a transgressive system tract (TST). The conglomerates are mainly classified as feldspathic litharenite. The porosity and permeability of the fan delta vary from 0.1% to 17.4% and 0.01 to 457.6 mD, respectively. The porosity and permeability vary from 2.02 to 16.2% and 0.02 to 237.2 mD in the nearshore subaqueous fan. Mechanical compaction, dissolution, and cementation together are noteworthy diagenetic processes. Compaction is the dominant factor in reducing porosity; cementation has a destructive effect, while dissolution improves reservoir quality. Feldspar dissolution leads to kaolinite, illite, dickite precipitation, and deteriorating reservoir quality. The probable sources of meteoric water flux in the deep-burial environments are related to the falling lake level during the HST. Fluid inclusion analysis revealed that quartz overgrowths were formed during the mesodiagenetic stage. The depositional environment and grain sizes are essential factors controlling reservoir quality. The effect of grain size and sorting coefficients on reservoir quality varies as a function of retrogradation and progradation sequences. The higher the porosity and permeability, the coarser the particle sizes in the retrogradation sequences. In the progradation sequences, poorly sorted sediments have higher porosity and permeability than well-sorted sediments. The braided channel embodies the best reservoir quality in the TST, while the distributary channel represents the best reservoir in the HST.The formation mechanism of the high-quality reservoirs is ascribed to the sedimentary facies, compaction resistance, and mineral dissolution. Overall, the conglomerate reservoirs developed in the HST are relatively better than those in the TST because they have greater labile fragments, resulting in more dissolution. This study demonstrated that for successful assessment of conglomerate reservoirs in the Lijin area and similar reservoirs elsewhere, reservoirs characterization must link sequence stratigraphy, facies, and diagenesis to understand the high-quality reservoir distribution to support the predrill evaluation of reservoirs.