Characteristics Of Unconformity Structure And Its Control On Oil And Gas Accumulation In The South Slope Of Dongying Sag

The south slope of Dongying sag is a low-angle monoclinal zone with north dipping,obvious stratigraphic overlap-denudation and developing several regional unconformities.This paper summarizes findings of previous researches and uses well-log and seismic data to determine the type of stratigraphic contact at the bottom of the Neogene and the Palaeogene within the research area,which will have significant meaning in analyzing the unconformity development characteristics.Based on fine interpretation of the seismic data,6 types are divided for unconformity?T₁?at the bottom of the Neogene on the profile,and 7 types are divided for unconformity?Tr?at the bottom of the Palaeogene on the profile.From the Boxing depression to the Luxi upheaval,the profile shows a transition from parallel-overlap truncation to parallel truncation and then to parallel poly-phase truncation.Considering the oil and gas discovered with the profile type together,the author suggests that,truncation and overlap unconformity can help oil and gas accumulate at the strata above and under the unconformity respectively.And the overlap-fault unconformity will transform the stratigraphic contact type at the bottom of the Palaeogene,which have strong ability in oil accumulation and is a good place for forming oil and gas connecting the upper stratum and the lower stratum.Unconformity is not only a plane,but a structure,which has three layers as the upper layer,the middle layer and the lower layer.The upper layer is mainly sand body and basal conglomerate formed by water-transgression,the middle layer is weathering clay mineral,and the lower layer is weathering-leaching mineral.After core observation,sample testing and processing based on the logging curves,a method for recognizing vertical structure of unconformity was proposed,which combines tri-porosity logging?P₂?,curve change rate and secondary porosity probability comprehensive index?ACD?.P₂ is obtained in the middle of the unconformity structure,which is different from the normal rock.The?AC??CNL??DEN has low value in middle layer of the unconformity structure,but with high value in the lower layer.From the ACD value it can be seen that there is obvious discontinuity between upper layer and middle layer of the unconformity structure,with high value on the contact surface,low and relatively stable value in middle layer of the unconformity structure,and with jagged fluctuation in the lower layer.The Neogene bottom's unconformity is in the south of Jinjia,and the Palaeogene bottom's unconformity has no middle layer structure at fault surface of the Shicun fracture,forming a dual structure.The structural layer thickness and the lithologic allocation relation control the oil and gas accumulation.An unconformity structure with the upper layer thickness exceeding 1m would be a good reservoir stratum,and a middle layer with a thickness exceeding 2m will have a capping function for the oil and gas.The upper layer of the unconformity structure develops with sandstone and conglomerate,with the lower layer developing sandstone and limestone.Such a lithologic allocation is most beneficial to oil and gas accumulation,and large amount of oil and gas has been discovered here.Typical anatomy for reservoir in unconformity at the bottom of the Neogene and Palaeogene shows that,the unconformity is not only a passageway for vertical and short-distance sideward migration of oil and gas,but also can form stratigraphic oil-gas reservoir with shielding of the structure's middle layer.In some areas with relatively thin or without middle layer,the oil and gas can migrate through layers,and forming overlap stratigraphic oil-gas reservoir in the upper layer.The Jinjia oil field and the Le'an oil field have obvious difference in their oil and gas reservoir forming mode.The Jinjia oil field forms reservoir by migration of weathering and leaching sand bodies in lower layer of the unconformity,and by capping function in the middle layer;Oil and gas in the middle part of the Caonan is formed in the upper and lower layers of the unconformity structure,and the upper and lower layers of oil and gas converge in the southern region;the Le'an oil field's buried-ridge area forms reservoir in the upper layer and lower layer of the unconformity concurrently through migration of the limestone in lower layer of the unconformity structure and of the dissolution-pore formed by weathering alteration.