Lithologic Reservoirs Identification And Prediction In Shida Depression, Jiuquan Basin

With the fast development of oil/gas exploration, those distinct structural oil fields have been found and current increasing of oil reservoirs worldwide is mainly from those old found oil fields. Therefore, preferential targets for exploration should focus on those hidden oil fields. It is very necessary to explore hidden reservoirs in outside area of those old oil fields. Of all the proven reservoirs, lith-stratigraphic oil reservoirs are taking high proportion, with lithological reservoirs increasing to 55% in 2003. It has become a new oil type in China.The Shida depression, located in the eastern Jiuxi depression of Jiuquan Basin, Gansu Province, with an area of 1300 km2, is a small-sized dustpan-shaped faulting depression. There is little work done in the Dahongquan depression, although the Shibei depression shows some signs of hidden oil reservoirs. Our oil/gas exploration indicated an existence of gas reservoir but not to an economic reservoir scale. The third resources evaluation showed a high occurrence of large geological reservoirs in this area, but the exploration and recognition is far from enough. Main problems are as follows: failure to understand the major oil-controlling factors and accumulation law of oil, distribution characteristics of sand bodies, and ambiguousness of hidden exploration targets. Due to the fact that almost no research has been done here, complicated geological setting, poor seismic data, variety of lith-stratigraphic oil/gas reservoir made it more difficult to predict the distribution of oil reservoirs.In this case ,the study begins with the fine structure interpretation and mapping in the targeted area in the Shida depression and obtained some new understanding of the faulting structures. Based on the stratigraphic theory, the study established an isochronous stratigraphic framework, in which the sedimentary system and facies of Cretaceous Chijinbao Formation are carried out. Combined with the geological characteristics of faulting basin and stratigraphy, this paper summarized the impacts of structural slope break over the type of lithological oil/gas reservoirs and distribution law in the Shida depression, and firstly discussed the control of structural slope break on the sedimentary facies zone differentiation and sand body types, and defined the underwater fan lithological reservoir as a major reservoir type. By integrating seismic, logging and geological study, this study made it clear that major exploration area can be classified the second- and third-level underwater coastal slope fans, and discovered three target fans. Using seismic facies, seismic attributes, 3-D visualization, hydrocarbon detection and other techniques, this thesis further analyzed the favorable and targeted zones in the depression, conducted predictable research on identifying lithological traps and lithological reservoirs from various aspects. Combined with the features of source-reservoir-cap rocks and above mentioned technical methods, the study made a comprehensive evaluation to properties of target fans and their oiliness. Finally, this study proposed one set of effective methods to analyze, identify and assess lithological oil/gas reservoirs, and this provides a referential guide for oil/gas exploration in dustpan-shaped faulted depression basins.