The Reservoir Characteristics And Main Controlling Factors Of The Xujiahe Formation In The West Of Yuanba Area Of Sichuan Basin

Basing on lithology, the Xujiahe formation in the west of Yuanba area of Sichuan basin could divide into six sections. They are the first section of Xujiahe formation, the second section of Xujiahe formation, the third section of Xujiahe formation, the fourth section of Xujiahe formation, the fifth section of Xujiahe formation and the sixth section of Xujiahe formation. The first section, the third section and the fifth section are main mudstone. The second section and the fourth section are main sandstone reservoir, and the sixth section is lacked. This paper, mainly through indoor and outdoor specific research and analysis, which include the observation of cores, sample preparation, the thin section analysis, physical properties analysis and cathodoluminescence analysis. Though researching the characteristics of those holes, which include the YB6hole, the YB22hole, the YB27hole, the YB204hole, the YB271hole, and so on, it shows that the lithology of reservoirs of this area are mainly quartz lithic sandstone, feldspathic litharenite, litharenite, and little quartz sandstone. The diameter of grains is ranged from0.21millimeter to0.35millimeter. The separation is medium. The psephicity is sub-angular to sub-round. All of those show low composition maturity and texture maturity, excepting small parts with high composition maturity. In this area, there are little clay matrix, carbonate cement and siliceous cement. The content of carbonate cements is greatly increased in the third section and the fourth section, which is mainly calcite cement. Inter-granular residual primary pore, secondary dissolution pore and micro-fracture are the main reservoir space. The secondary dissolution pore is widespread, which include inner-granular dissolution and inter-granular. The micro-fracture is developed in the third section and the fourth section, which include structural fracture and diagenesis fracture. Regarding it as a whole, the physical property is poor, with average porosity of3.15%and average permeability of0.042X10-3μ m2. Studying diagenesis of the sandstone reservoir of the second section of Xujiahe formation in the west of Yuanba area shows that it experienced three stages, involving the stage of buried diagenesis, the stage of early diagenesis and the stage of middle diagenesis. It is in the B stage of middle diagenesis. In process of diagenesis of the sandstone reservoirs in this area, controlling factors are mainly connect with those diagenesis as follow. The main destructive diagenesis of compaction and some cementation result in most primary porosity lost. Some types of cementation, such as the development of quartz overgrowth and the precipitation of authigenic chlorite, are considered to be retentive diagenesis due to their important role on the preservation of porosity under deep-bury condition though they occupy some pore space. The development of dissolution pore and micro-fracture, greatly increased and improved the reservoir space and the reservoir quality, which considered as constructive diagenesis.