Overpressure Evolution Caused By Hydrocarbon Generation In Petroliferous Basins

Overpressures which the fluid pressures exceed hydrostatic values are common in many Petroliferous Basins. Many mechanisms can contribute to the development of overpressures include compaction disequilibrium, hydrocarbon generation, aquathermal pressuring, the release of fluid during dehydration reactions, and tectonic compression. The overpressure evolution plays an important role in hydrocarbon generation, migration an accumulation and there are different evolution processes for overpressure generation by different mechanisms. In the extensional tectonic environment of basins, compaction disequilibrium and hydrocarbon generation are the most potential mechanisms for overpressure generation. In this paper, overpressure evolution caused by hydrocarbon Generation was performed in four steps:(1) reconstruction of hydrocarbon generation overpressure model; (2) confirm the oil generation can be the major mechanism of overpressure generation; (3) hydrocarbon generation modeling based on the reconstruction of the burial, thermal and maturity history; (4) determine the time of hydrocarbon charge and evolution of the overpressure regime through PVT modeling using fluid inclusion data; (5) calculate the overpressure evolution cause by hydrocarbon generation in Dongying and Baiyun depression by using the reconstructed overpressure model caused by hydrocarbon generation. The following conclusions can be drawn.1. Oil generation can be the major mechanism of overpressure generation in petroliferous basin because the strong overpressure can be developed during the oil generation which is evidenced from physical simulation experiments of oil generated overpressure. Hydrocarbon generated overpressures are generated in the source rocks when the rate of volume increase generated by the transformation of high-density organic matter to oil is more rapid than the rate of volume loss by flow. Therefore, oil generated overpressure model was constructed with consideration of the change of oil density during the compaction, the compaction of water and kerogens due to the oil generated overpressure, the leakage of oil and water during the oil generation, the effect of hydrogen index on oil generation and oil expulsion. A very good correlation between the calculated overpressure caused by oil generation using the oil generate overpressure equation and physical simulation experiments of oil generated overpressure, whicl indicate that the oil generated overpressure equation is reliable and can be used to study the overpressure evolution caused by oil generation. Among the TOC (Total Organic Carbor Content), hydrogen index and the residual coefficientαresponse to overpressure, the oil residua coefficientαdisplays the more sensitive to oil generated overpressure than TOC and hydroger index. The hydrogen index show the less sensitive to oil generated overpressure than TOC Overpressure can not be generated if the residual coefficientαis less than 0.75 during oi generation of the source rocks. Gas generated overpressure model was constructed based or taking into account the leakage of natural gas and water during the hydrocarbon generation, the thermal cracking of oil to gas, the effect of hydrogen index on hydrocarbon generation, the compaction of water and kerogens due to the hydrocarbon generated overpressure, the change of oil, gas and water density during the compaction, the natural gas solution in oil and water, the C2+in natural gas is liquid for the high pressure condition and hydrocarbon expulsion. Among the TOC (Total Organic Carbon Content), hydrogen index and the natural gas residual coefficientβresponse to overpressure, the natural gas residual coefficientβhave the less sensitive to hydrocarbon generated overpressure than TOC and hydrogen index and the hydrogen index show the more sensitive to hydrocarbon generated overpressure than TOC. Overpressure can generate when the natural gas residual coefficientβis over 0.2 during hydrocarbon generation for the source rocks.2. The Dongying Depression of the Bohai Bay Basin is a Cenozoic basin with a prolific oil-producing province in China. The gray to black mudstones, calcareous mudstones and oil shales in the third and fourth members of the Eocene Shahejie formation has been considered as the important source rocks which are dominated by typeⅠkerogens with high TOC contents. Overpressure has been identified in the source rocks formation with pressure coefficients up to 1.99 by the drill stem test (DST) and well log data. The sonic log displays the obvious response to overpressure in Dongying Depression because overpressured mudstones have higher acoustic travel time values than the normally pressured mudstones for a given depth and the acoustic travel-time of normally pressured and overpressured mudstones decrease with the vertical effective stress increasing. The resistivity and density data displays no obvious response to overpressure. the disequilibrium compaction can not be the main cause of overpressure generation in Dongying Depression because overpressured sediments are normal compaction which are evidenced from overpressured mudstones without anomalously low density, the apparent lack of correlation between density and effective stress, overpressured sandstones without anomalously high matrix porosities and geothermal gradient. Oil generation is the main origin of overpressure for the mudstones in Dongying Depression. the observed specific characteristics are:(1):the pressuring phase of overpressured reservoirs in the Es3 and Es4 formations of Dongying Depression is predominantly oil and oil-bearing, overpressured water reservoirs are rare; (2):the depth to the top of overpressure is ranged from 2000m to 3000m and increases with the burial depth of the source rocks, with the temperature of approximately 87-123℃and an estimated vitrinite reflectance (Ro) of 0.5-0.75%; (3):The organic-rich source rocks in Es3 and Es4 formations of Dongying Depression are still capable of oil generation; (4): Natural microfractures are widespread in the low-permeability source rocks of Es3 and Es4 formations in Dongying Depression; (5):Calcareous mudstones in the Es3 formation can reduce the seal rock porosity and permeability to form a mudstone pressure seal. Overpressure in the sandstones is generated by pressure transmission resulting from the overpressured fluids which expel from the source rocks of Es3 and Es4 formations in Dongying Depression charging into the reservoir rocks by means of active faulting and fracturing. Maturity and hydrocabon generation histories modeling indicate:①the oil generation for the Es4s source rocks began from about 37Ma and peak oil generation occurred at 32Ma; oil generation for the Es3s source rocks began from about 35Ma and reach peak oil generation at 2Ma.②oil generation in Dongying depression can be summarized into two stages, the first stage occurred at approximately 37-20 Ma and the second stage is from 5Ma to the present.③the Es4s source rocks have the high transformation ratio up to 100% at present and the transformation ratio of the Es4s source rocks in the most of the study area is over 60%. The transformation ratio for the deepest source rocks reached 10%,60% and 90% at 35Ma,32Ma and 27 Ma respectively. The transformation ratio of the Es3x source rocks is ranged from about 10-90% at present. The deepest source rocks transformation ratio reached 10% and 60% at 29Ma and 5 Ma respectively. Many oil inclusions with the fluorescence colour of yellow or near yellow and blue or near blue are observed in the sandstone samples of Dongying depression. The wavelength at max of the fluorescence spectra for the oil inclusions with the fluorescence colour of yellow or near is over 500nm, oil inclusions with the fluorescence colour of blue or near blue, the wavelength at max is less than 500nm. The value of CIE_X increases continuously with the value of CIE_Y increasing and the wavelength at max increases with the value of CIE_ X and Q increasing gradually, which indicate that the maturity and composition for the oil inclusions in the northern of Dongying depression change continuously. Two episodes of hydrocarbon charge have been determined in the sandstones of the Es4s and Es3x in northern of Dongying depression:the first charge took place between at 24 Ma and 20Ma and the second charge occurred between at 3 Ma and 2Ma. The gas/oil ratios of 68 oil inclusions in 12 sandstone samples in Donying depression are ranged from 2.6% to 14% at 20℃using confocal laser scanning microscopy (CLSM) and increase with the homogenization temperature increasing. Paleopressures of individual petroleum inclusions are modeled based on measuring the composition of oil inclusions with the fluorescence colour of yellow and blue using the analysis method of MSVV which was developed by CSIRO petroleum lab in Australia. Five types of overpressure evolution for the reservoir rocks in the Es3 and Es4 of Dongying depression are identified by combination the origin of the overpressure. Type 1:the overpressure is developed in the reservoir rocks by the second oil charge until at present; Type 2:the overpressure generation in the reservoir rocks by the second oil charge is not strong and the reservoir rocks belong to the normal pressure at present because of the leakage of oil and water Type 3:the overpressure is not developed in the reservoir rocks by the second oil charge until a present; Type 4:the overpressure is developed in the reservoir rocks by the first oil charge until a present; Type 5:the overpressure is developed in the reservoir rocks by the first oil charge and overpressure release occurred from the end of the Ed formation deposition to the end of the Nrr formation deposition to cause the decreasing of overpressure because the leakage of fluids overpressure is generated again at the end of the Nm formation deposition due to the second oi charge until at present. Three types of overpressure evolution for the source rocks in the Es3x and Es4s of Dongying depression are identified using the oil generated overpressure model. Type 1: the overpressure is generated 25Ma ago and the source rocks belong to normal pressure until at present because of the oil expulsion at the end of the Ed formation deposition; Type 2:the overpressure is generated at 25Ma ago and after 2 Ma; Type 3:the overpressure begin to generate at the end of the Nm formation deposition. The overpressure evolution caused by oil generation for the sections indicate that there were three episodic stages of overpressure generation and release for the source rocks in the Es3x and Es4s of Dongying Depression. The first episodic stage of overpressure generation and release occurred before the end of the Ed formation deposition, the second stage is from the time of the Ng to Nm formation deposition and the third stage is from the end of the Nm formation deposition to OMa.3. Baiyun Depression is the biggest depression in Pearl River Mouth Basin and has good exploration foreground for the natural gas. The coal and mudstones in Enping formation are the major source rocks for the natural gas which are dominated by typeⅡ2 andⅢkerogens. It is confirmed that the present formation pressures are normal even in the deepest subsiding center of the Baiyun Depression by the drill stem test (DST), well log data and seismic interval velocity calculations. However, wide distribution of gas chimneys was discovered in the Baiyun Depression according to the 2D seismic wipeout features. Bright spots are found in the top or both sides of the diapiric structures and have a positive relationship with the nature gas reservoir that is confirmed by the drilled wells in the north of the Baiyun Depression. Hot fluid intrusions occurred in the diapiric structure as evidenced from elevated vitrinite reflectance (Ro) values, high levels of smectite-illite transformation, and elevated fluid inclusion homogenization temperatures, which is caused by the vertical migration of the high temperature and pressure fluid through faults within the diaper structures. The Enping source rock in the western sag, the middle sag and the eastern sag of the Baiyun depression begins to generate hydrocarbon about at 20Ma,30Ma and 22Ma, reaches the hydrocarbon generation peak about at 10 Ma,22 Ma and 8 Ma, reaches the high maturity about at 8Ma,17Ma and 1Ma respectively. The transformation ratio of the Enping formation source rocks in the most area of the western sag are below 80% at present and the transformation ratio reached 5%,30% and 45% at 14Ma, 10Ma and 5 Ma respectively. In the middle sag, the transformation ratio of the source rocks in Enping formation is over70% at present and reached 5%,60% and 90% at 25Ma,15Ma and 5 Ma respectively. The transformation ratio of the Enping formation source rocks in the most area of the eastern sag is over 50% at present and the transformation ratio reached 5%,10% and 45% at 16Ma, 10Ma and 5 Ma respectively. The overpressure evolution caused by hydrocarbon generation for the source rocks of Enping formation in Baiyun depression are divided into two episodic stages of overpressure generation and release by the calculation of overpressure using gas generated overpressure model. The first episodic stage of overpressure generation and release occurred before the Dongsha tectonic movement and the second stage is from 5 Ma to 0Ma. The overpressure generation in the western sag, the middle sag and the eastern sag of the Baiyun depression begins at 15Ma,30Ma and 19Ma.