INFLUENCE OF SYNDEPOSITIONAL VOLCANISM ON OLIGOCENE SANDSTONE DIAGENESIS AND QUALITY: A COMPARISON OF THE BZ34-9 AND KL6 FIELDS IN THE SOUTHERN BOHAI SEA, CHINA

The BZ34-9 and KL6 fields are located in the north and south of the Laibei Low Uplift in the southern Bohai Sea, east of China. Despite similarities between Oligocene fluvio-deltaic sandstone sequences in these two fields, predicting quality reservoirs has remained a challenge. In particular, the impact of regionally extensive syndepositional volcanism, recognized principally by basalt flows/dikes and minor tuffs, on the diagenetic history and ultimately, reservoir quality has been elusive. Here we combine analysis of 3D seismic data with detailed reservoir characterization using well log analysis, sandstone petrography, scanning electron microscopy, and X-ray diffraction techniques to evaluate the influence of volcanism and related extrusive rocks on reservoir quality of the Shahejie and Dongying formations in these fields. Seismic amplitude and variance cube thresholding methods were used to determine the spatial distribution of volcanic rocks and phases of volcanic activity. Volcanic overflow facies are characterized by continuous, high amplitude reflectors, while volcanic channel facies are discontinuous and weak. For the BZ34-9 area, braided river deltas were deposited on a gentle slope at low depositional rates. Syndepositional basalt flows occur in the upper and lower parts of the sand body and high-quality sandstone reservoirs are characterized by dolomite coats, zeolite cementation and dissolution, and mixed-layer clay coats, which collectively had a positive impact on the reservoir quality. By contrast, the KL6 area was situated on a steep slope and fan delta deposition occurred at a high rate. The Oligocene basalts are located in the center of the basin, down dip, and thus, had limited influence on sandstone reservoirs, which are characterized by abundant kaolinite and carbonate cements. However, Oligocene strata in the KL6 area are situated near Tan-Lu Fault Zone, which is associated with Neogene basaltic volcanism and hydrothermal activity. Such activity caused recrystallization of siderite, sericitization, and precipitation of ferrodolomite cements. This study shows that the impact of volcanism on sandstone reservoirs is variable and controlled by the timing of eruptions, depositional setting and rates, and the configuration of the sand bodies and basalts.