Characteristics of the low-pressure spatial and temporal distributions of oil- and gas-bearing layers in the Ordos Basin, China

Abstract

Abnormal pressure conditions and spatial variations have direct and powerful influences on the aggregation, dissipation, accumulation and development of hydrocarbons under subsurface conditions, and they are extremely important aspects that must be focused on for safe coal mining and oil and natural gas development. The Ordos Basin, located in the western part of the North China Craton (NCC), contains significant reserves of oil, gas, coal and other mineral resources and is a typical low-pressure basin. This paper conducts an overall analysis and comprehensive comparison of the spatial and temporal variations in the pressure coefficients of oil and gas areas in the Ordos Basin. The analysis is based on a dataset comprising 589 sets of pressure coefficient–depth data from hydrocarbon layers that are uniformly classified in the basin's main oil and gas fields. Notably, high-pressure features develop in the basin only in the gas fields located in the centres of areas with high gas generation intensity, in individual sections of lithological seals and in the gas layers beneath the gypsum salts of the Ordovician Majiagou Formation. The Mesozoic oil layers and Paleozoic gas layers throughout the basin, which are buried at depths up to 4500 m and within a stratigraphic era spanning 3 × 10⁸ years, generally dominate low pressures, and these abnormal pressure features are not noticeably affected by spatial or temporal variations. Additionally, the ranges of the pressure coefficients and the distributions of the peak values of the oil and gas layers in the upper-oil and lower-gas areas (UOLGAs) generally resemble each other. The pressure coefficients for gas reservoirs only slightly exceed those for oil reservoirs. These unique features are rare in many large- and medium-sized petroleum bearing-basins worldwide. Excluding the high abnormal pressure data of the subsalt gas layer in the basin, based on the magnitudes, proportions, and distributions of the pressure coefficients of the gas layers, as well as the geological background, it is possible to classify the 10 gas fields in the basin into five distinct categories: (1) weak low-pressure dominated; (2) weak low-pressure dominated, mixed with high-pressure; (3) weak low-pressure dominated, supplemented with normal pressure; (4) normal and weak low pressures coexisting; and (5) abnormally high pressure dominated. Regarding the spatial distribution, the pressure coefficients of the gas layers exhibit a macroscopic pattern of gradual increase from the northern to the southern regions. Furthermore, the unique characteristics of the generally low pressures in the basin, in the coalfield (mine) gas pressure coefficient, in the gas content, and in the gas explosion frequency correspond to the performance and can be corroborated by each other. This study provides a scientific basis and theoretical foundation for a profound understanding of the accumulation (mining) effects of evolution-modification and the unique features of hydrocarbons and coal seams (mines and fields) in the Ordos Basin, which can help to promote and guide the further exploration and development of energy minerals in different regions of the basin; furthermore, a new type of pressure system has been added to global energy basin pressure systems.