Reservoir Characteristics of Upper Paleozoic Tight Sandstone Gas in the Southeast of the Ordos Basin

Nianxi Wang, Hongxia Cao, Xiao Yin, Jinsong Zhou, Qianping Zhao, Ting Shang, and Juan Qiang
Research Institute of Yanchang Petroleum Group Co. Ltd, Xi’an, China

Tight sandstone gas is the slow flow speed in sandstone, they display low porosity, permeability, gas saturation and high water saturation. It is unconventional gas in general condition. Based on careful analysis of outcrop sections, the data of rock cores, thin sections, SEM, physical properties and pore structure, reservoir characteristics and influence factors in the southeast of the Ordos basin.

Beach-barrier coast deposit system in Benxi formation was consisted by barrier island sand dam, lagoon, sand flat, mixed flat and mud flat micro-facies, and the barrier distributed as moniliforme trending near N-S trend along the coast. The meandering river delta system was established in the Shanxi formation, which consist of under distributary channel, distal bar, frontal sheet sand and interdistributary bay. The constructive braided river delta sedimentary system was established in He-8 member, which consist of under distributary channel and interdistributary bay.

The reservoir is chiefly grey mid-fine grained sandstone, slightly gritstone and conglomerate in partial layer section. Rock type is mainly lithic sandstone, lithic quartz sandstone and quartzose standstone. With decreasing quartz and increasing debris content from the Benxi formation to the He-8 of Xiashihezi Group, interstitial matter has limited difference in each region. Heterobase composition is chiefly kaolinite, hydromica and chlorite, ement composition is mainly siliceous, ankerite and ferrocalcite, cementation type is primarily porous cement and basement cement. Supporting way is mostly grain-supported, matrix supported and point-line contact forms. The study indicates that sandstone is a good rock composition maturity and medium-good texture maturity, moderate sorted and well-sorted, re-angular and re-round round forms.

The reservoir space is dominated by secondary pore, which can be subdivided into intragranular dissolved pore, intracrystalline pore and farthing crack. The reservoir was characterized by lower permeability and lower porosity with poor physical property. The poor pore structure parameter displayed mainly small pore throat, poor grain sorting, fine skewness, high displacement pressure and median pressure, and the dissolution improving throat connectivity is limited With vertical comparing of the reservoir property, the Benxi formation displacement pressure accounted for 69% less than 1MPa and mean maximum open pore is 1.86μm, which was relative batter than Shihezi and Shanxi formation, they were 25%, 23.5%, 0.44μm and 0.61μm separately. The reservoir is closely controlled by sedimentary micro-facies and diagenensis, of which subaqueous distributary channel sands and barrier sand dam are the most favorable micro-facies, and the strong compaction pressure solution is the main cause of low porosity and permeability.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013