Controls on reservoir quality in fan-deltaic conglomerates: Insight from the Lower Triassic Baikouquan Formation, Junggar Basin, China

摘要

Fan delta sandy conglomerates form important oil and gas reservoirs. Recently, a new giant oilfield was discovered in the Lower Triassic Baikouquan Formation (T(1)b) in the Mahu Sag of the Junggar Basin, northwestern China. However, the fan delta sandy conglomerate reservoirs are strongly heterogeneous. Most have poor permeability and host little hydrocarbons, while a subset at the delta front exhibit high porosity and permeability, forming high-quality reservoirs. To explain this disparity, the sedimentary facies, diagenesis, and reservoir quality of the sandy conglomerate strata, and their interactions, were studied. Detailed logging of 253m cores indicates that the T(1)b was deposited in a lacustrine fan delta complex, comprising ten lithofacies from coarse matrix-supported pebbly conglomerate (Gm) to fine massive mudstone (Fm) in four Facies Associations. Interpretation of optical microscope, backscattered and scanning electron microscope imaging show that these ten lithofacies underwent differential diagenesis, especially dissolution of minerals such as orthoclase. Dissolution was particularly prevalent in the subaqueous distributary channel deposits of sandstones (St), hyperpycnal-flow massive sandstones (Sm) and grain-flow deposits of sandy gravelly conglomerates (Gcs) at the delta-front, forming abundant secondary pores. These pores enhanced the reservoir performance and led to high porosity and permeability within these three lithofacies. For these reservoirs, continuous winnowing at the delta front led to relatively high primary porosities, and subsequent mineral dissolution during burial diagenesis, closely related to acidic Mn-rich hydrocarbon-bearing fluids, significantly improved the porosities further. Migration of hydrocarbon-bearing fluids was facilitated by faults cutting through the reservoir and underlying source rocks. This led to the development of belts of high-quality reservoir at the delta front near the fault conduits. This model for reservoir development and charge suggests that further reservoirs can be expected within the T(1)b of the Mahu Sag.