Impact of sequence stratigraphy, depositional facies, diagenesis and CO2 charge on reservoir quality of the lower cretaceous Quantou Formation, Southern Songliao Basin, China

The fourth member of the Lower Cretaceous Quantou Formation in Southern Fuxin Uplift zone of the Southern Songliao Basin is typical tight oil reservoir which has recently been explored and produced. For effective exploration and development, the evaluation and control factors of the reservoir quality of the K(1)q(4) sandstones should be thoroughly studied. The study focused on the control of the combination of sequence stratigraphy, depositional fades and diagenesis on the reservoir quality, and identified the impact of CO2 charging, based on the core and thin section observation, XRD, SEM, fluid inclusion and isotope testing and analysis. The fourth member of the Lower Cretaceous Quantou Formation consists of lowstand system tracts (LST), transgressive system tracts (TST) and highstand system tracts (HST). The K(1)q(4) sandstones were mostly deposited in distributary channels and crevasse splays of a delta plain within the LST and TST, and underwater distributary channels and sheet sands of a delta front within the HST. The K(1)q(4) sandstones, mostly silt to medium grained, medium to well sorted, and are mostly lithic arkoses and feldspathic litharenites, have undergone significantly diagenetic alterations, such as compaction, quartz cementation, unstable grains (feldspar and volcanic fragments) dissolution, carbonate cementation and clay minerals cementation. The reservoir quality of the K(1)q(4) sandstones are poor with low porosity and permeability, showing variations among the different system tracts and depositional fades, and are influenced by the charging of CO2 and controlled by the combination of sequence stratigraphy, depositional fades and diagenesis. The sandstones with high reservoir quality are mostly deposited in high energy environments, including the (underwater) distributary channel sandstones, especially the LST distributary channel sandstones with the best reservoir quality among the LST, TST and HST sandstones, which can be ascribed to larger grain size and better sorting resulting in larger intergranular pore space after compaction, and less content of carbonate cements, illite and mixed-layer I/S, and more pore-lining chlorite and more effectively dissolution. The poor reservoir quality of sandstones from crevasse splay and sheet sand is due to the small grain size and bad sorting which resulted in extensive compaction and the resulting weak dissolution. The charging of CO2 has dual impact on the reservoir quality of the K(1)q(4) sandstones. The CO2 charging can induce the dissolution of unstable detrital grains and the precipitation of dawsonite. The former create new secondary pore space, while the latter occupy the intergranular pore space. The CO2 charging is harmful to the reservoir quality when the dawsonite content exceeds a threshold of 5%.