Evolution of chlorite composition in the Paleogene prototype basin of Jiyang Depression, Shandong, China, and its implication for paleogeothermal gradient

Abstract

The Dongying Basin, Huimin Basin, and Zhanhua Basin constitute the Jiyang Depression in Shandong Province. They are major oil and gas exploring districts within the depression. Through reconstructions of the paleotemperature of the three basins facilitated with the chlorite geothermometry, the thermal history of the Paleogene prototype basin in Jiyang Depression and its geologic significance were explored. This study reveals that the Si 4+ component in chlorites reduces gradually as its buried depth increases, while the Al IV component increases accordingly. The chlorite type changes from silicon-rich diabantite to silicon-poor ferroamesite and prochlorite. The prochlorite in this district only appears in the deep buried depth, high temperature, and relatively old stratigraphies; while the diabantite appears in the shallower buried, low temperature, and newly formed strata; the ferroamesite exists in the conditions between prochlorite and diabantite formation. The diagenetic temperatures of the chlorites in these Paleogene basins are 171-238℃ for the Dongying Basin, 160-202℃ for the Huimin Basin, and 135-180℃ for the Zhanhua Basin. The differences of the chlorite diagenetic temperatures in the three basins were controlled by the duration time of the structural depressing processes. Higher temperature indicates longer depression time. The relationship between the chlorite diagenetic temperature and its buried depth indicates that the average paleogeothermal gradient is about 38.3℃ /km in the Paleogene prototype basin of Jiyang Depression. It was higher than the present geothermal gradient (29-30℃/km). This phenomenon was attributed to the evolution of the structural dynamics in the depression basin.Paleogene, Jiyang depression, chlorite component, chlorite geothermometry, paleotemperature, paleogeothermal gradient Basin's thermal history is crucial for the reconstruction of basin's structural evolution history; for oil and gas-rich basin, the thermal history is proved to be useful to inferring how the hydrocarbon resource rocks produce hydrocarbon and how the oil and gas were transported and gathered. The reconstruction of a basin's thermal history is based on the application of different geothermometric methods [1] . At present, the most popular and effective geothermometric methods are vitrinite reflection ratio (R o ) and apatite fission trace (AFT). R o method has been widely used as a geological thermometer for a long time on studying the maturity of hydrocarbon resource rocks and the burying history and the thermal evolution [2][3][4][5][6][7] . Vitrinite is solid organic substances and coal layers that are distributed in sedimentary rocks. Its reflection ratio (VR o ) increases generally as its thermal evolution extents or its buried depth increase, while its resolution decreases as its coalification goes on [1] . Therefore, its application is restricted. The principle of AFT is that the fission trace of apatite retreats as its temperature rises. It is an ideal geological thermometer

Quartz Thermoluminescence—Another Potential Paleo‐Thermometer for Studies of Sedimentary Basin Thermal History

“…As the quartz TL equivalent temperature displays the mean thermal regime structure effects in one certain geological time, it is plausibly derived that the geo-temperature of relevant formations of Shahejie might keep decreasing since later Paleocene. Zhao et al [26] also proved that the burial paleo-temperature in Paleogene should range from 171 • C to 238 • C based on illite composition analysis, which was also much higher than the bottom drilling temperatures given by the oil exploration company [24,25] . Also, the equivalent temperature study in this work supports the tectonic thermal evolution history of the Dongying depression as evidenced by the forward modeling from Qiu et al [27,28] .…”