Characteristics, origin and controlling effects on hydrocarbon accumulation of overpressure in foreland thrust belt of southern margin of Junggar Basin, NW China

Abstract

Aiming at the differential distribution of overpressure in vertical and lateral directions in the foreland thrust belt in the southern margin of Junggar Basin, the study on overpressure origin identification and overpressure evolution simulation is carried out. Based on the measured formation pressure, drilling fluid density and well logging data, overpressure origin identification and overpressure evolution simulation techniques are used to analyze the vertical and lateral distribution patterns of overpressure, genetic mechanisms of overpressure in different structural belts and causes of the differential distribution of overpressure, and the controlling effects of overpressure development and evolution on the formation and distribution of oil and gas reservoirs. The research shows that overpressure occurs in multiple formations vertically in the southern Junggar foreland thrust belt, the deeper the formation, the bigger the scale of the overpressure is. Laterally, overpressure is least developed in the mountain front belt, most developed in the fold anticline belt, and relatively developed in the slope belt. The differential distribution of overpressure is mainly controlled by the differences in disequilibrium compaction and tectonic compression strengths of different belts. The vertical overpressure transmission caused by faults connecting the deep overpressured system has an important contribution to the further increase of the overpressure strength in this area. The controlling effect of overpressure development and evolution on hydrocarbon accumulation and distribution shows in the following aspects: When the strong overpressure was formed before reservoir becoming tight overpressure maintains the physical properties of deep reservoirs to some extent, expanding the exploration depth of deep reservoirs; reservoirs below the overpressured mudstone cap rocks of the Paleogene Anjihaihe Formation and Lower Cretaceous Tugulu Group are main sites for oil and gas accumulation; under the background of overall overpressure, both overpressure strength too high or too low are not conducive to hydrocarbon enrichment and preservation, and the pressure coefficient between 1.6 and 2.1 is the best.