Study on the Experienced Highest Paleotemperature and Thermal Maturity Evolution of the Lower Paleozoic Marine Shales in the West of Middle Yangtze Region, Central China

Zhang, Jiankun; He, Sheng; Yi, Jizheng; Hou, Yuguang

The Lower Paleozoic shales of marine strata in the western region of the middle Yangtze of central China are considered to be high degrees of thermal evolution. It is, however, very difficult to determine their thermal maturities (vitrinite reflectance, Ro) due to no vitrinite in organic macerals in the marine shales of the Lower Paleozoic Formations. The objective in this study is to restore the experienced highest paleogeotemperature and then to reconstruct thermal maturity evolution history of the Lower Paleozoic marine shales in the west of the central Yangtze area using basin modeling technology with the data from thermo-acoustic emission experiments, bitumen reflectance measurements, kerogen FTIR (Fourier transform infrared spectroscopy) analysis. The results of thermo-acoustic emission experiments illustrate that the Lower Paleozoic marine shales have a good thermal Kaiser effect and the rock thermo-acoustic emission signals vary with geotemperatures regularly. The experienced highest paleogeotemperatures of the Lower Cambrian strata change from 293.3 degrees Celsius to 324.4 degrees Celsius, and the highest paleogeotemperatures of the Upper Ordovician-Lower Silurian strata change from 210.4 degrees Celsius to 256.5 degrees Celsius. Furthermore, the sensitivity of thermal Kaiser effect has been tested by reheating the rocks experiments, suggesting the thermo-acoustic emission can be applied to the measurements of the marine shale experienced highest paleogeotemperatures. Combined with the measured highest paleotemperatures and referred to the bitumen reflectance values as the constraints, the thermal maturity modeling of Lower Paleozoic marine shales has been carried out, the results of the thermal maturity modeling demonstrate that the modeled values of vitrinite reflectance (Ro) for the Lower Cambrian and Upper Ordovician-Lower Silurian marine shales range between 3.5% and 4.0% and 2.5% to 3.0%, respectively. The organic matter maturations in the Lower Cambrian and Upper Ordovician-Lower Silurian marine shales are in the stage of high thermal evolution with over-maturities, consistent with the characteristics of kerogen FTIR analysis in the Lower Paleozoic Marine Formations.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013