Origin and Evolution of Overpressure in the Changji Depression, Junggar Basin, Northwest China

Abstract

Overpressure is widespread in the Permian, Triassic and Jurassic formations and regionally in the Cretaceous of the Changji Depression with the pressure coefficient from 1.24 to 2.07 and excess pressure from 11.5 to 56.5 MPa. The depth to the top of overpressure ranges from 4500 m to 5310 m. The study shows hydrocarbon generation is the main origin of overpressure in the mudstones and the specific characteristics are as follows: (1)The present burial depth of overpressured mudstones are more than 4500 m exhibiting no anomalously low density and high primary porosity. The measured porosity of 12 overpressured mudstone samples from different depth ranges from 2.56% to 8.45%. However, the measured density does not change much(2.51 ~ 2.58 g/cm³). (2)The source rocks at the top of overpressure with the vitrinite reflectance values of 0.75% ~ 0.9% are in the main hydrocarbon generation stage and capable of maintaining an abnormally high pore pressure. And the distribution of the top of overpressure consists with the Jurassic source rocks. (3)The overpressured reservoirs are predominantly oil saturated or oil bearing without water saturated and natural microfractures are widespread in the source rocks that may be the pathways for episodic expulsion of hydrocarbons and overpressure dissipation and thus causes overpressure in reservoirs. Two episodes of hydrocarbon charge have been determined in the Jurassic and Cretaceous sandstones by the fluid inclusions. Two types of oil inclusions with the color of blue-white and little yellow were observed and two different phases of aqueous inclusions likewise have been identified according to the homogenization temperature(95 ~ 110 °C and 110 ~ 125 °C). In the light of burial, thermal and hydrocarbon generation history, it indicates the two hydrocarbon charges occurred at the end of Cretaceous (70 Ma) and Paleogene (23 Ma) respectively. Then based on the model Guo established before it is programmed to reconstruct the overpressure evolution in the mudstones caused by hydrocarbon generation. The result shows the overpressure evolution in Changji Depression is a multi-stage process. There were three episodic stages of overpressure generation and released twice for the J1b source rocks; the J1s source rock experienced two episodic stages of overpressure generation and released only once; the hydrocarbon evolution of J2x source rock is very low so that the overpressure increased from about 110 Ma ago to present with no relief.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015