Oil & Gas Geology ›› 2017, Vol. 38 ›› Issue (1): 49-61.doi: 10.11743/ogg20170106

• Petroleum Geology • Previous Articles     Next Articles

Micro-petrology, pore throat characteristics and genetic mechanism of tight oil reservoirs-A case from the 6th and 7th members of Triassic Yanchang Formation in the Ordos Basin

Zhong Dakang1,2   

  1. 1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;
    2. College of Geosciences, China University of Petroleum, Beijing 102249, China
  • Received:2016-07-20 Revised:2016-12-20 Online:2017-02-28 Published:2017-02-27

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Abstract:

The 6th and 7th members of Yanchang Formation (Chang 6 and Chang 7) contain a large number of tight oil in the Longdong area of Ordos Basin. However,poor understanding of their microscopic characteristics and genesis has hindered the exploitation of tight oil. The current study attempts to investigate the pore throat characteristics and genesis of tight oil reservoirs of Chang 6 and Chang 7 members of Yanchang Formation on the basis of pore cast thin section,field emission electron microscopy (FE-SEM) and so on. The results show that the tight oil reservoirs were deposited in distal bar and sheet sand of delta front and gravity flow of semi-deep to deep lake,and the tight oil reservoir rocks with fine grain size and high matrix content (8%-10%) consist of very fine sandstones,siltstones,pelitic siltstones and silty mudstones. These reservoir rocks have very poorly developed pores,low thin section porosity averaging at 1.8%,small pore radius averaging at 30 μm,fine throat radius averaging at 0.08μm,low porosity averaging at 9% and low permeabi-lity of less than 0.3×10-3 μm2. The main pore types of the tight oil reservoirs are micropore of intergranular matrix,dissolution pore of feldspars and rock fragments and intercrystalline pore of cements. The microscopic characteristics of different oil reservoir rocks are different. The low-energy sedimentary environment is decisive in determining the fine grain size and high matrix content of the reservoir sandstones,which have weak resistance to compaction in early burial stage,hence,significant amount of original pore volume is lost,and the pore throat sizes become smaller. In the reservoir rocks with tiny pore throats,the strong carbonate and clay mineral cementation (especially the bridging and network-like cementation of illite),caused by semi-permeable membrane effects of ions,further reduce the porosity and permeability of the reservoir rocks,preventing the later acidic fluid dissolution and formation of secondary porosity. In addition,the strong replacement of quartz by detrital micas grain,hydromuscovite matrix and carbonate cement lowers the resistance of the reservoir rocks and increases the solid volume,finally leading to the tightening of the reservoir rocks.

Key words: microscopic characteristic, formation mechanism, reservoir, tight oil, Ordos Basin

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