The 1'st members of Yingcheng formation, lower Cretaceous in Xingcheng development area, Songliao Basin mainly have rhyolite gas reservoirs. The 1'st member of Yingcheng is belong to the low-porosity and low-permeable reservoir, the reservoir space are mainly fracture-pore. By the means of field out crops observation, core description, thin section analysis, well logging identification and the seismic forecasting, the fractures of 1'st member of Yingcheng volcanic reservoir was analyzed in three aspects: the genetic, the attitude and the width. The dominate fracture type of 1'st member of Yingcheng are the tectonic related fractures(78.5%), then the diagenetic related fractures(17.0%), corroded fractures(4.2%) are the last. The high angle fractures and the vertical fractures are most common in tectonic related fractures; the oblique fractures are the following. The dominate type of diagenetic related fractures are level fractures, followed by high angle and oblique fractures. The fractures in the reservoir were mainly wide fractures (width>1mm) and narrow fractures (0.1mm<width<1mm), the fractures of middle width are the minority. The distributions of the fractures were mainly controlled both by the regional stress field and the activity of the faults, the fractures are displayed by belts in coincident with the strike of the main faults.
Keywords: volcanic gas reservoir, reservoir fracture, structural fractures, diagenetic fractures, high angle fracture
The development and exploration of volcanic gas reservoir is now the hot spot in petroleum fields. The volcanic gas reservoirs have been discovered in Songliao basin, Chaidamu basin, Sichuan basin and so on[1-2]. It is quite difficult to study the fracture characteristics of volcanic gas reservoir. This paper takes the volcanic gas reservoir of the Yingcheng Formation of Xushen area in Daqing oilfield for example to study the fracture of volcanic reservoir.
The volcanic gas reservoir of the Yingcheng Formation in Xingcheng area is located in Shengping-Xingcheng structural belt of Xujiaweizi Fault-Depression in the north of Songliao basin, mainly developed in the first member of Yingcheng Formation in Lower Cretaceous Series. The buried depth of reservoir is 3000~3500m. The volcanic gas reservoir is mainly the rhyolite. The types of reservoir are mainly fracture-porosity style, which belongs to low porosity and low permeability reservoir.
1. Lithology and lithofacies characteristics
The lithology and lithofacies characteristics control the development of fractures and cavity, which is the dominant factor to make the types of reservoir space and reservoir heterogeneity complicate[3-4]. By the chemical analysis of core gathered from 7 wells, the chemical composition of volcanics is mainly the SiO2(74.24% on average) and next is Al2O3(10.4% on average) and K2O+Na2O(8.42% on average). The figure of alkaloid-SiO2 is made by the analysis ,which suggests that the lithology in the area is mainly acidic volcanic rock and the next is the intermediate lava. The type of rocks is mainly the rhyolite and others are the mixpah, trachydacite, dacite, andesite and necrolite.