A Comparative Study of Silicified Rock Reservoir in Shunnan 4 well of Tarim Basin with Hydrothermal Chert Reservoir of Parkland Gas Field in Western Canada Sedimentary Basin

Abstract

The silicified limestone reservoirs in the Yingshan Formation of Middle Ordovician, discovered from SN4 well in 2013, have made a high yield in Shunnan area which is located in the center of Tarim Basin, northwest China. Comparing to previous work about the silicificaition, much more detailed petrographic descriptions have been carried out in this study. The most important diagenetic features to the silicification are identified including replacement quartz, quartz cement (characterized by 4 textures: equant quartz, bladed quartz, overgrowth and quartz cement in later fractures) and three types of calcite cement (CC1-CC3). Geochemical analysis including Si-O isotope analysis on quartz and C-O isotope analysis on calcite cement are processed, together with fluid inclusion studies on both quartz and calcite cement. Back to the discovery of this unique type of reservoir worldwide, the only comparable one is the hydrothermal chert reservoir interbedded among dolomite and dolomitic mudstone in the Wabamun Group of Upper Devonian, discovered in 1956 in Parkland, NW Western Canada sedimentary basin (WCSB). Due to limited reference in this chert reservoir, detailed work including core and thin section observations have been processed for further understanding. Two types of replacement dolomite and two types of dolomite cement occurring in Parkland comprise fine crystalline dolomite (RD1), medium-coarse crystalline dolomite (RD2), medium-coarse crystalline dolomite cement (CD1) and saddle dolomite cement (CD2), respectively. The chert mainly replaced the original host rock or filled in the fractures/dissolution vugs after the saddle dolomite cement. A third type of silica identified in this study is the euhedral quartz cement. Coreii based geochemical exploration has been put forward including C-O and Sr-O isotope analysis (mainly on RD1, RD2, CD1, CD2 and calcite cement). Fluid inclusion analysis on the saddle dolomite and euhedral quartz cement has also been processed. Based on the geological setting, the Upper Devonian reservoirs in Parkland show many similarities to those of Ordovician in SN4 well, including original limestone framework and the structural controlled fault systems. The differences mainly display as a pervasive dolomitization before chertification in Parkland, while in the silicified intervals of SN4 well, the dolomite mainly represents as the cement along with stylolites. The textures of the silica which contribute to the reservoir quality in both areas are also dramatically different. Chertification in Parkland formed from Tournaisian to late Viséan at a depth around 3200 m, while in SN4 well, the silicification depth was rather deeper reaching 6600 m and estimated began in the Late Devonian. The silicified reservoirs in both areas have made breakthroughs in the gas production. A better understanding of the nature and the forming mechanism of these silicified carbonate reservoirs is of critical importance to guide further exploration in both of the areas, which could also be applied in other areas with similar background setting.