Diagenetic sequence stratigraphy of the Carboniferous Donghe Sandstone in the Tazhong area

Abstract A diagenetic sequence stratigraphic study was conducted on the Carboniferous Donghe Sandstone Member in the Tazhong Area. Based on data from drilling, well logging, and seismic profiles, the Donghe Sandstone Member was subdivided into one third-order sequence, within which the lowstand (LST), transgressive (TST), and highstand (HST) systems tracts were identified. The LST was determined to consist of fluvial deposits, the TST of tide-dominated shoreface and estuarine deposits, and the HST of wave-dominated shoreface and alluvial fan deposits. This study investigates the diagenetic sequence stratigraphy of the Donghe Sandstone by analyzing reservoir pore characteristics and their formation mechanisms, pore distribution patterns, the distribution of carbonate cements, as well as the variations in formation water chemistry and the distribution of organic acids. Diagenetic processes associated with the sequence boundaries include freshwater dissolution, carbonate cementation, and freshwater infiltration. The dissolution processes notably involved the leaching of feldspars and the concomitant precipitation of kaolinite. Within a 20-m interval adjacent to the sequence boundary, feldspar content decreases significantly while kaolinite content increases. Reservoir quality near the sequence boundary is enhanced due to the secondary porosity generated by freshwater dissolution of feldspars. Relatively low δ 18 O and δD values observed near the sequence boundary provide evidence for meteoric water infiltration.