Quantitative reconstruction of formation paleo-pressure and case studies
Formation pressure governs the generation, expulsion, migration, accumulation and preservation of petroleum. Fluid-rock interactions during diagenesis and mineralization are also affected by the formation pressure. Thus, investigating the formation paleo-pressure in sedimentary basins is an important aspect of research into the mechanisms and processes related to hydrocarbon accumulation, and it plays an increasingly important role in hydrocarbon exploration and prospective prediction.
Formation pressure occurs during the long-term evolution of basins, and is regulated by tectonism, deposition, diagenesis, fluid flow, geothermal field, and magmatic activity. Oil and gas exploration is increasingly directed at deep, ultra-deep and ancient strata which, however, have generally experienced multiple stages of tectonic movements. Reconstruction of formation paleo-pressure in these strata is not easy.
Various methods have been established for paleo-pressure reconstruction in sedimentary basins, including approaches based on basin modelling, fluid inclusion analysis, differential stress of rocks, transformation of clay minerals, acoustic transit time of mudstones, and seismic wave velocity. Each of these methods has advantages and limitations, but most can only determine the formation pressure in a certain geological period, rather than the whole pressure evolution process. Furthermore, some of these methods were established based on simple porosity evolution models, which are not applicable to reservoirs with complex fluid flows, intense tectonic activities, and abnormal porosity evolution paths.
Origins of abnormal pressures typically change during geological history. In this study, a new method is proposed for reconstructing the paleo-pressures in strata by integrating various paleo-pressure calculation methods according to the identification of the formation mechanism and the main factors responsible for controlling abnormal pressures. According to the geological background, quantitative analyses of the factors that might control overpressure were first conducted to clarify the contributions of each mechanism during different geological periods. Pressure evolution was reconstructed by fluid-compaction modelling with constraints imposed by paleo-pressures obtained from fluid inclusions or differential stress methods. Determining the mechanisms responsible for overpressures during geological history is the basic prerequisite for paleo-pressure research. Thus, quantitative studies were conducted of the contributions of disequilibrium compaction, gas charging, oil cracking, temperature reduction, and tectonic uplift and subsidence to overpressures.
Three case studies of paleo-pressure reconstruction were performed for the Sinian strata in the Sichuan Basin, Ordovician strata in the north uplift in the Tarim Basin and the Permian strata in the Sulige Gas Field in the Ordos Basin, where these three study sites are normally pressured, weakly over-pressured and abnormally low pressured at present, respectively.
The Sinian formation in the central Sichuan Basin is mainly normally pressured at present. Under the constraint of the trapping pressures due to fluid inclusions in three periods, which were calculated using PVTsim software, the evolution of pressure in the Dengying Formation was obtained by basin modelling with a fluid-compaction coupling model. Pressure in the Sinian Dengying Formation is due to a combination of hydrocarbon accumulation, oil cracking to form gas, and temperature reductions caused by tectonic uplift, where these different factors played dominant roles during diverse periods.
The Sulige Gas Field in the Ordos Basin is a typical gas field with an abnormally low pressure. A great temperature reduction from 165°C to 105°C occurred in the last 100 Ma. Regardless of gas dissipation, pressure would be decreased by 17.7-22% when temperature declined by 50-60°C. On the other hand, gases were dissipated 17-24 vol%, resulting in 23-32% decreases in the formation pressure. Based on fluid inclusion analysis and numerical modelling, weak overpressures occurred twice throughout the geological history, where the first overpressure was generated at 195 Ma, which was suspended by the uplift at 160 Ma before, overpressure was generated again after 140 Ma, where it was maximized in 98 Ma at a greatest depth of 4425.6 m and with a pressure coefficient value of 1.1. Subsequently, both the formation pressure and pressure coefficient decreased gradually due to uplift and denudation, and changed to an abnormally low pressure with a coefficient of 0.85 at present.
The fluid pressure during the critical period of tectonic compression can be quantitatively calculated using the differential stress method with calcite twins as a paleo-barometer. The paleo- pressure in the Ordovician Yinshan carbonate strata in the Shunnan Area of the Tarim Basin was reconstructed as a case study. The Shunnan Area was in a tectonic compression environment from the middle and late Caledonian to the Hercynian period, and the orientation of the principal stress changed from SW-NE to SE-NW. The paleo-stress ranged from 66.15 to 89.17 MPa, with an average of 82.06 MPa in the Caledonian, and ranged from 56.97 to 79.29 MPa, with an average value of 66.80 MPa in the Hercynian. The excess fluid pressure in the carbonate strata during tectonic compression deformation was calculated by the difference between realistic effective vertical stress and theoretical vertical stress. Combined with modelling results, a weak overpressure was developed in the Ordovician strata during the middle to late Caledonian period by the strong compression related to the Paleo-Kunlun Ocean subduction. The formation pressure gradually deceased to normal pressure because of strata uplift and stress field conversion. Another two phases of overpressure were formed at the end of the Permian and Neogene periods, originated from southeast-trending compression and gas filling, respectively.
Pressure analysis is the basis of fluid dynamic system analysis, which is significant for hydrocarbon migration and reservoir diagenesis. The development of effective paleo-pressure recovery methods for carbonate strata may be essential for addressing various problems in deep and ultra-deep layer pressure research.