Research On Preservating Conditions Of Gas In Deep Intelval Of Jiyang Basin

The deep layer is currently distributed at burial depth of >3500 meter in Yiyang basin, and the main explorative object is finding cracking gas in lower Tertiary. New researches have obtained many important results, especially in the well of Xinlishen 1, so the natural gas exploration of deep layer has great potential in Jiyang basin. A good preservation of trapped hydrocarbons is a main controlling on oil/gas accumulation. Quantitative study on preservation ability is very significant for the natural gas exploration of deep layer.Based on a large amount of data obtained from geology and experiment, a study of caprock sealing, fault sealing and hydrodynamic conditions has been made, with the application of theories from multi-disciplines, such as geology, mathematics and mechanics. The mathematical models for resistance of caprock and gas potentical energy of reservoir have been built. An evaluation method of the sealing ability of caprocks is put forward by the difference between the resistance of caprock and gas potentical energy of reservoir. The sealing ability of caprocks is very significant for the preservation and destruction of trapped hydrocarbons not only for the present, but also for the time after the oil/gas accumulated. With the use of acoustic logging data, this paper analyzed the sealing ability of poorly-compacted mudstone and its evolvement, and effectiveness of sealing ability has been explained. Fault sealing ability is studied through normal stress of fault surface, lithology face-to-face of two plates, and muddy contents in faulted belt.The analysis results show that the fault sealing ability is main factor impacting oil/gas accumulation in Jiyang basin. Source rocks, caprocks and faults are the main factors of controlling the oil/gas reservoir forming in faulted basin. The time-space matching relation of the source rocks, caprocks and faults control the oil/gas accumulation size and the distribution scope. By analyzing the time-space matching relation of the source rocks, caprocks and faults, their effect on oil/gas reservoir forming and distribution was studied in Jiyang basin. The following important progresses have been extended.(1)The mudstone caprocks of the deep part in Shahejie formation developed well and have better sealing abilities. The sealing ability and the lateral continuity of caprocks of Es1, Es3 and Es4 members are good so they can serve as caprocks for the underlying gas reservoir. The regions of having medium or good sealing ability of caprocks are Lijin-Minfeng subsag, Niuzhuang subsag, Bonan and Wuhaozhuang subsag.(2) Based on the mechanism of the percolation mechanics , the gas leakage through caprock can be divided into two stages: the displacement and the percolation. When the residual pressure in the gas reservoir is less than the critical displacement pressure in the caprock, the gas reservoir is covered in a static state; conversely, it is in a dynamic state. The loss amount of natural gas in the dynamic state lies on the leakage loss amount of natural gas, so the caprock thickness, the displacement time and the percolation time play very important roles in gas leakage. This paper, therefore, developed a quantitative evaluation model for the gas leakage, and estimated the sealing ability of caprocks of lower Tertiary in Jiyang basin.(3) Based on the analysis of formation and evolution of overpressure,the evolution law for sealing of overpressured mudstone caprock was quantitatively studied. The result showed that sealing of overpressured mudstone caprock evolved in stages.Each release of overpressure indicated the end of an old evolution stage and the beginning of a new evolution stage.Sealing of overpressured mudstone caprock increased gradually in each evolution process, and it was the worst at the release period of overpressure.The study of evolution for sealing of overpressured mudstone caprock can not only determine exactly sealing ability and forecast sealing evolution,but also study sealing efficiency for oil/gas accumulation. With the use of acoustic logging data,, this paper computed the release times and the release depth of overpressure of Es3 member in Dongying depression,and estimated the sealing ability of overpressured mudstone caprock of Es3 member and its'controlling on oil/gas accumulation. The results are as following:â‘ the beginning period of pressure sealing formation of Es3 member is earlier than the period of the main hydrocarbon expulsion of source rocks of deep layer, and the sealing ability of mudstone caprock of Es3 member for oil/gas accumulation is of efficiency;â‘¡the overpressured mudstone in Es3 member released one times,and it is in the second evolution stage now. The sealing of overpressured mudstone caprock increases with increasing overpressure;â‘¢the sealing ability of caprock of Es3 member is good , with 8.0 MPa overpressure, and it is one of the main controlling factors on oil/gas accumulation in deep layer.(4) Based on the fault sealing mechanism, this paper studied the sealing ability of main faults in Jiyang basin through normal stress of fault surface, lithology face-to-face of two plates, and muddy contents in faulted belt. The fault intensity was studied depended on the growth index and fault activity rate. The results indicate that the faults in Jiyang basin have strong sealing abilities. The fault intensity was strong during the sedimentating of Es3 and Es1 members,and died out during the sedimentating of Guantao formation. The time-space matching relation of the source rocks, caprocks and faults is good, and it is favorable for oil/gas from deep layer to accumulate and preservate.(5) Accompanying the origin and evolution of sedimentary basin, hydrodynamic field underwent a process of origin, development and perished. By studying the hydrodynamic conditions and the palaeohydrogeogical parameters, the hydrodynamic field in the Dongying depression was symmetrical,with the characteristics of gravity-induced centripetal flow recharged by meteoric water along the edges of the depression, centrifugal flow formed by compaction in the central depression area, and cross-formational flow and concentration by evaporation in the southern slope and northern fault ramp along the basin margins and the central fractural zone in the interior depression area. The intensity of hydrodynamics had the property of vertical zoning. The intensity weakened gradually with the increasing depth. The deep layer of Dongying depression has steady hydrodynamic conditions, with the CaCl2 water type, and it is favorable for natural gas accumulation.Based on the researches on the sealing ability of caprock, the sealing ability of fault ,the hydrodynamic conditions and the data of source rocks and reservoirs, this paper estimated the favorable areas for natural gas accumulation in Jiyang basin. The west and east portions in North Steep Slope of Dongying depression and the Bonan subsag are favorable for natural gas accumulation and preservation. Main exploration orientations are the lithology traps distributed widely in the gas-generating center and the margin,palaeostructural and structural-lithological traps developed below the lower Es3 member around the gas-generating center.