Geological Interpretation And Hydrocarbon Detection Related To The Paleozoic Coal-bearing Series Reflections In The Ordos Basin

The Hangjinqi area is one of the main exploration blocks of Sinopec in the Ordos Basin.The Palaeozoic Taiyuan Formation,Shanxi Formation and Lower Shihezi Fotmation are the major exploration layers.Due to the the existing of coals in Taiyuan and Shanxi Formation,the seismic exploration is asscioated with coal-bearing reflection.In addition,the block suited on the northern part of Ordos Basin,the coal-bearing stratas are thiner than that in the central area of this basin,The internal coal seams of Shanxi Formation not only produce strong reflection shielding the reflection information of Shanxi formation,but also generate interferes with the bottom reflection of the Lower Shihezi Formation.For the specific seismic data in the eastern part of Hangjinqi block,the coal reflections poss its own particularity and are exactly equivalent to the general reflection of coal measures.A series of collapse reflections having been identied in coal reflections,two peaks and two troughs and three peaks and three troughs reflection having occurred under the same condtion that the cumulative thickness of Taiyuan and Shanxi Formation are larger than half wavelength point,this two special reflection phenomenals are not specifically proposed in previous studies and their geneis are unknown.In terms of the current exploration status,the exploration of gas reservoirs in hangjinqi area has accomplished sedimentary facies analysis,fault evolution analysis,reservoir-formed model analysis and main control factors analysis of hydrocarbon accomulation,having entered the stage of model summary.Thus,in the case of a relatively clear understanding of gas reservoir geology,it is of great practical significance to conduct hydrocarbon dectection of the main gas resevoirs of the H1 member of the Lower Shihezi Formation.Nevertheless,due to the influence of coal-measures reflection,it is a challenging task to carry out the research of the hydrocarbon detection through the combination of the seismic reflection effected by the overlying coal-measures reflection and the gas testing data already existed.The collapse structures present on seismic sections are found in the Ordovician carbonate,the Carboniferous Taiyuan Formation and the Permian Shanxi Formation.The plane distribution of collapse structures is characteristiced by the coherent attribute,curvature attribute and throw attribute,the collapses in the width of 100 to 200m,and annular fault aslo could be found.Local carbonate rocks have collapsed and embedded in the basement of the basin.Majority of the faults formed by collapse terminated in the H1 member and have thickened this member,suggesting the collapse occurred in the early of the Permian.The bottom of the collapse strutctures are corresponding to the columnar refecletions,weakness zones(blind faults and rifting center)in basement,moreover,tuff have been found in the coring of the H1member in the periphery drillings.Therefore,the collapse structures are collapse by magmatic activity.The occurance of karst collapse pillars would lead coal seams damage,and the amplitude of coal reflection would decrease,but the coal-measures reflection in the collapse locations are alwalys marked by strong reflection.Accordingly,the possibility of karst collapse pillars is excluded.In terms of the merging and bifurcation of coal reflection,the coal seam thicknesses of the Taiyuan and Shanxi Formations and the spacing between the two sets of coal are considerd as the major roles in the merging and bifurcation of events through a combination of well-data statistical analysis and creation of crossplots.Seismic forward model and seismic detectable resolution confimed the boundary conditions of events variation.The formation of three peaks and three troughs requires that the thicknesses of the two sets of coal are greater than 2 m.The spacing between the two sets of coal must also exceed 10 m with a detectable lateral extension,which would otherwise generate two peaks and two troughs.The distance(spacing)between coals in the Taiyuan and Shanxi Formations is proportional to the bottom sand thickness of Shanxi Formation,but large spacing may be accompanied by thick sand,which leads to a decrease in coal thickness.Consequently,the two peaks and two troughs indicate a combined relationship between the thin sandstones at the bottom of Shanxi Formation and the thick coalbeds in the Taiyuan and Shanxi Formations.The three peaks and three troughs suggest a combined relationship between the thick sandstones at the bottom of the Shanxi Formation and the thin coalbeds in the Taiyuan and Shanxi Formations.On the basis of the foreward model of events merging and bifurcation,the coal reflection effect on the H1 member has been investegated through the presence or absence of the coal seam at the bottom of the model,two different expressions indicating the influences of the coal seams reflection.The T9d peak of the H1 member is submerged by coal seam reflection.Fortunately,the trough above the the T9d peak can reveal some usefull information of the resevoirs of H1 member.Based on the statistics and comparison of the instantaneous amplitude spectrum and integral amplitude curves of the gas-testing layers in 11 gas-bearing and 6 non-gas-bearing exploratory wells,the isofrequency amplitude and integral amplitude values of the gas-bearing wells between 10 Hz and 25 Hz are much greater than those of the non-gas-bearing wells.Hence,four spectral attributes of the H1 membe were extracted and selected to repsent the spectrum differences between gas-bearing and none-gas bearing regions,including the isofrequency amplitude at 10 Hz,the integral amplitude at 25 Hz,and the low-and high-frequency attenuation gradients.Subsequently,the comparison shows that these four spectral attributes are better than the attributes extracted along trough above T9d peak.Finally,the threshold values of the selected spectral attributes were determined by the comparison of gas-bearing and non-gas-bearing wells,and gas detection was accomplished using multiple-attribute crossplots.Meanwhile,in order to improve the relaiablity of hydrocarbon detection evidence,analysised AVO attribute using pre-stack gather data,and the AVO gradient attribute which is suitable for the gas-bearing response of the H1 member were extracted.The lithologic inversion was conducted in the face of the wave impedance superposition problem between sandstone and mudstone,and the sandstone distribution of H1 member and Shanxi Foramtion was predicted.Exploration shows that the hydrocarbon in the study area migrated very limitided and accmuliated autochthonous.The H1 member is dominated by lithologic gas reservoirs,and the Shanxi group is dominated by structural gas reservoirs.The massive collapse strucutres identied in this stuys are good path for hydrocarbon migration.Under the favorable configuration of coal seam thickness,reservoir to strata ratio and migration paths in the study area,an area of about 20km~2 in the southern part of the study area were recognized by the combination of the results of spectra difference hydrocarbon detection and AVO gradient attribute.The drilling results clarified that the wells situated at the two peaks and two troughs regions is accompanied by higher gas productivity compred with wells situated at the three peaks and three troughs regions.That is to say,the investigation of favorable exploration target of the Shanxi Formation is foucused on the recoginition of the area that two peaks and two troughs regions overlapping with anticline structures,and in which there is no collapse structures offset the top of Shanxi Formation.