Formation Mechanism and Distribution of Paleogene-Neogene Stratigraphic Reservoirs in Jiyang Depression

摘要

During Paleogene - Neogene period, multiple scale unconformities had beenformed in Jiyang depression, which provided favorable conditions for stratigraphicreservoirs. In recent years, various Paleogene-Neogene stratigraphic reservoirs inJiyang depression have been found, and proved reserves were rising significantly,which fully showed a great exploration potential for this kind of reservoirs. But the practice of exploration in recent years indicated that the unconformities carrier systemand its ability of sealing, petroleum migration and its accumulation model,distribution of stratigraphic reservoirs are uncertain, which deeply restrict the exploration degree of stratigraphic reservoirs in Jiyang depression.
　　 Based on the analysis of a large number of exploration wells and seismic data fortypical reservoirs, the paper analyses unconformities construct and its effect togeneration in the Paleogene - Neogene, and summarize the distribution pattem ofstratigraphic reservoirs based on petroleum mechanism and accumulation model.Finally, a highly quantitative prediction modelof height of pools in stratigraphicreservoirs was established, the research results effectively guided the explorationpractice of stratigraphic reservoir.
　　 There are four macro unconformity types of Paleogene- Neogene formationwhich including truncation-overlap, truncation-parallel, parallel-overlap and paralelunconformity in Jiyang depression. Besides truncation-overlap unconformity lies inslopes of depression, and parallel unconformity developed inside of depression,another two types lie in local areas. Unconformity can be developed verticallythree-layer structure which including unconformity roof rock, weathered clay layerand semi-weathered rock. It also can be two-layer structure if without weathered claylayer. And part of semi-weather rock can be form a hard shell accuse ofits fillingprocess during the later stage. Geological characteristic of the structure layer ofunconformity is different in lithology, mineralogy, element geochemistry and weatherdegree index. Based on optimal partition of sequential number and principalcomponent analysis, logging quantification recognition method about unconformitystructure layers was established, on which effective identification of unconformitystructure layers can bu achieved in the case of no rock core. The formation of variousunconformity structure types is related to many factors such as, parent rock lithology,interval of deposition hiatus, palaeotopography, and preservation conditions, which aretogether to control spatial distributions of unconformity structure types.
　　 Macro styles and its vertical structure of unconformity can be effected as a blocking, reservoir, trap or carrier system. Blocking affection to fluid depends on weathered clay layer, hard shell of semi-weathered rock and mudstone. So petroleummigration and accumulation units is relatively :independence above and below unconformity if structure layers mentioned above existed. Reservoir affection is dueto permeable rock, including roof sandstone, semi-weathered sandstone, semi-weathered carbonate rock, semi-weathered igneous rock and semi-weathered metamorphic. Trap-controlling affection related to macro unconformity type and its juxtapose to permeability and impermeability rock above and below unconformity. It is easy to develop stratigraphy traps where the permeability and impermeability beds juxtapose in a truncation-overlap unconformity, where up permeability and down impermeability in parallel-overlap unconformity, and down permeability and up impermeability beds juxtapose in a truncation-parallel. Transporting affection is owing to lateral continuity of permeable rock of unconformity. In a terrestrial rift basin, petroleum migration in transverse or vertical short distance in local area ,andis not conducive to petroleum long distance along unconformity,because interbedding pattem of mudstone and sandstone is dominated, and its physical property of mudstone improved poorly.
　　 Because of the long distance from resource to trap, migration and accumulationprocese is very complicated.Accumulation process of Paleogene-Neogene stratigraphic traps can be summarized as following: allochthonous source rock, compound transportation,later period charging, buoyancy and pressure conversion driving for accumulation ,and blocking by non-permeable layer of unconformity.Trap types and its distribution are controlled by unconformity structure styles.Petroleum distribution and its scale are controlled by generating ability of source rock. Petroleum accumulation area is decided by positive tectoruc units. If carrier system existed, oil column of stratigraphic reservoirs is effected by four main factors which including generation expulsion quantity, migrating distance, dip angle and capillary resistance of carrier layer. Based on the analysis of single factor, the prediction model of height of oil columu through multi-factor regressions wasestablished. Based on the model, the paper definited 6 favorable areas, whichreserves in these areas exceed 1.5 x108t.
　　 Research results of the paper combined closely with exploration practice, andaccording to previous research results, 31 exploration wells had been drilled, whichof them 17 wells were successfully from 2006 t0 2009. There is accumulationproved reserves was up t0 2362x104t. and predict reserves was t0 3684x104t.