| The exploration of lithologic reservoirs needs not only the guidanceof theory, but also the strong support of modern exploration techniques.The predecessors have proposed “source control theory”, multiple oil andgas accumulation zone theory, delta front facies-belt controlled theoryand lithologic reservoir exploration theory in faulted basin for theexploration of onshore oil and gas reservoir in China. The rapiddevelopment of the contemporary borehole, seismic and drillingtechnologies greatly improved the discovery of oil and gas reservoirs inChina.Based on core, well logging and seismic data, combined with theanalysis of sedimentary system and tectonic system in Jiergalangtu Sag,Erlian Basin, this paper discussed the distribution law of reservoirs in thisarea, and systematically studied the identification techniques forlithologic reservoirs. The result shows that the lithologic reservoirsdistribution in this sag is obviously dominated by sedimentary facies beltand slope break belt. A set of prediction methods for deep-water reservoirwas proposed, which effectively eliminates the interference of semi-deepand deep lacustrine calcareous mudstone, argillaceous dolomite oneffective deep-water reservoir. Based on the different geologic models,the relation chart among the four properties (lithology, physical property,electric property and hydrocarbon-bearing property) was established,which extremely raises the success rate of lithologic reservoirsexploration in this area.1. There developed different sedimentary systems in different structural location in Jiergalangtu Sag. In the study area, theremainly developed six types of sedimentary facies, including fan delta,braided river delta, nearshore subaqueous fan, sublacustrine fan andlacustrine facies, and different types of sand bodies control thedistribution of subtle reservoirs.(1) Steep-slope nearshore fan deposition is mainly of high-densityturbidity currents. It is mainly composed of coarse clastic sediments,which developed in lacustrine dark mudstone, and formed the frequentrhythmic deposition of glutenite, conglomerate, aleurolite and mudstonein the section. The steep-slope subaqueous fan can be divided into threetypes of subfacies, including inner fan, middle fan and outer fan.(2) Sublacustrine fan is fan clastic sedimentary body caused bydeep-water gravity current, and it is more in deep water areasgeographically. It may be formed by the direct injection of flood gravitycurrent into deep water area, and may be formed by the slumping andrapid accumulation of delta front sediments. The sublacustrine fan hascomplex lithologies and multiple types of rocks, and the typical lithologyis characterized by dark gray and black mudstone mixed with matrix-supported glutenite.(3) Fan delta front sheet sand distributed widely, and frontdistributary channel glutenite is its important type of reservoir rock. Thelithologies include variegated massive conglomerate, glutenite andpebbled sandstone, as well as gray and dark gray mudstone.2. The slope break belt is highly developed in Jiergalangtu Sag.Its control effect on the sandstone reservoir in faulted lake basin isquite outstanding, and the control effect on oil and gas is also obvious.The different location and different types of the slope break beltresult in obvious difference of sand control and reservoir control.The sag can be divided into steep slope faulted slope break belt inthe northwest and gentle slope faulted slope break belt in the southeast.The steep slope faulted slope break belt is mainly controlled by thecontemporaneous faults and companion faults in the northwestern marginof the sag. There developed sublacustrine fan and slope fan,simultaneously stratigraphic overlap exists, which can form strata, fault block, fault nose and lithologic pinchout traps.Gentle slope faulted slope break belt includes levelⅠ and Ⅱfaultedslope break belts and level Ⅲ flexure slope break belt. The levelⅠfaultedslope break belt roughly covers the area in the south of Ji-17well andJi-61well, and it is also located on the high part of the level II and levelIII slope break. Its sedimentation is mainly controlled by paleotopographyand it is easy to form stratigraphic unconformity traps and stratigraphicoverlap traps. The level II faulted slope break belt is dominated by manyNE oriented normal faults. The fault displacement is large, and indifferent position and different fault, the intensity of activities varies. Theformation thickness of fault upper wall and foot wall changes obviouslyand there exist downcutting filling in the belt. Because of thedevelopment of the fault, the traps are mainly of fault block and faultnose. Meanwhile, there also exist stratigraphic overlap, denudation andpinchout, and they can form stratigraphic overlap traps, lithologic updippinchout traps and structural-lithologic traps. In addition, from thereservoir calibration in seismic section, they are far from the oil sourcerocks. That why they can form oil reservoir is greatly related to the deepfaults and unconformable surface which serves as lateral oil and gasmigration pathway. Beside the sedimentary paleogeomorphology wherethey exist, the level III flexure slope break belt is also related to theboundary faults in the north. As the boundary faults constantly sink, theslope areas relatively rise, and the gradient increases, and therefore formsthe flexure slope break belt. In this kind of slope break belt, fault is notdeveloped, the depositional landforms are mainly large gradient slope,and there mainly developed lithologic traps and fault nose traps grownclose to the level II faulted slope break belt.3. A set of prediction methods for deep-water reservoir wasproposed, effectively solving the interference of semi-deep and deeplacustrine calcareous mudstone, argillaceous dolomite on effectivedeep-water reservoir prediction.The first member of Tenggeer Formation (especially the VI, VII,VIII sand group) deposited in semi-deep and deep lacustrine environment,and the calcareous mudstone and argillaceous dolomite developed well and stably distributed. When mudstone contains calcium, its intervaltransit time reduces obviously, and sometimes it is even lower than that ofsandstone. If we use the conventional wave impedance inversion methodfor3D space description of deep-water reservoir sand bodies, it is boundto cause a great multiplicity. Based on the different types of reservoir rockelectricity and seismic reflection characteristics, this paper summed up amodeling method for reservoir prediction under deep lacustrineenvironment, proposed an effective reservoir detecting method by usingnatural gamma ray inversion method, and solved the multi-solutionproblem caused by the simple use of wave impedance to distinguishsandstone from calcareous mudstone and argillaceous dolomite, throughbaseline offset correction, glutenite scale value correction, speciallithology point-to-point editing correction processing. Explorationpractice shows that the technology is effective in predicting the effectivereservoir in deep sublacustrine fan and nearshore subaqueous fan.4. The relation chart among the four properties (lithology,physical property, electric property and hydrocarbon-bearingproperty) was established, which extremely raises the success rate oflithologic reservoirs exploration in this area.The electric properties are the integrated response of lithologies,physical properties and hydrocarbon-bearing properties. Due to the fastvertical and lateral variation of lithologies and physical properties, theelectric properties vary greatly. We established two types of relation chartamong the four properties for glutenite reservoir and pebbled sandstonereservoir respectively, which extremely raises the success rate oflithologic reservoirs exploration in this area. |
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