Tracing deep fluids in the Bozhong Sag: Evidence from integrated petrography and geochemistry data

Abstract

Abstract The Bozhong Sag is a mature exploration area for petroleum in the Bohai Bay basin, China, which is a typical oil-bearing basin and hosts the Penglai (PL) 19-3, and Qinhuangdao (QHD) 32-6 giant oil fields (Xue, 2018). Despite 60 years of hydrocarbon exploration in the basin, only few large gas fields have been discovered. The discovery of the Bozhong (BZ) 19-6 gas field not only indicated that the Bozhong Sag has great potential for natural gas exploration, but also raised questions regarding the formation of large-scale natural gas fields in typical oil-prone basins and in sapropelic source rocks. The present study therefore performs a detailed investigation of the sources and migration pathways of the deep fluids in the Bozhong Sag area as a means of shedding light into this scientific challenge. The Bozhong Sag is located in the thinnest crust and tectonic transition zone of the Bohai Bay basin. Intense extension and thinning of lithosphere led to the upward ejection of deep crust and mantle materials under diapirism. Improvements in this tectonic setting and the fluid migration path occurred as a result of early faults that were formed by compressive thrust in the Indosinian-Yanshan orogenies in addition to a process of extensional development and reactivation of Himalayan faults. We obtained cores from 13 wells in the Bozhong Sag, and subsequently observed their petrography and analyzed samples for fluid inclusions. Also, volcanic rock samples were analyzed for major and trace elements, and carbonate rock samples were analyzed for δ13C, δ18O, and 87Sr/86Sr. Previously reported δ13CCO2 and He isotopic data were are also discussed. Our results showed that petrography was characterized by thermal fading and hydrothermal breccia around fractures filled with quartz and calcite. Deep fluid activity was mainly reflected by petrographic assemblages of quartz, ankerite, pyrite, plus calcite and volcanic eruptive rocks that were i) enriched with large ion-lithophile elements (LILE), ii) enriched with light rare-earth elements (LREE), iii) relatively deficient in high-field strength elements (HFSE), and iv) relatively deficient heavy rare-earth elements (HREE). Thermal anomaly events were reflected by the homogenization temperature of fluid inclusions, and characteristics of magmatic hydrothermal fluids were exhibited by δ13C, δ18O, and 87Sr/86Sr. Our findings indicated that deep fluids were mainly derived from the deep parts of the upper mantle, and that they were slightly contaminated by crustal materials. Combining our results with those of previous studies on deep xenoliths and xenolithic fluid compositions in eruptive rocks of eastern China (Zhang et al., 1999; Han et al., 2008), we interpreted that deep fluid properties in the study region were characterized by high H2 and low total volatile gases. We conclude that the deep fluids in the Bozhong Sag was mainly derived from deep material in the upper mantle, and that the fluid activity pattern was aided by the deep fracture-fault systems in addition to central and fissure-type wide-area eruption activities.