The Storage Space of Chang7 Shale, Yanchang Layer in Ordos Basin and Exploration Potential for Shale Oil
Xiaobing Niu1,2, Shengbin Feng1,2, Yuan You1,2, Fang Wang1,2, Xiaowei Liang1,2, and Weidong Dan1,2
1National Engineering Laboratory of Exploration and Development of Low-Permeability Oil-Gas fields, Xi’an, China
2Exploration and Development Institute of PetroChina Changqing Oilfield Company, Xi’an, China
In the paper, several features of Chang7 shale in China, Yanchang layer, Late Triassic, Ordos basin, were studied, such as storage space, oil bearing property, content of brittle minerals, etc. On this basis, the potential for shale oil exploration in continental limnic shale systems, just as Chang7 shale, has been discussed. Chang7 shale can be divided into three types, Source shale, Mud shale and Silty shale, according to sedimentary structure, rock constituents, and logging parameters, including gamma ray, density and deep investigation induction resistivity. The demarcation value of those parameters was calibrated by TOC. According to statistical results of Chang7 shale, the average TOC of Source shale is 13.72%, the accumulative thickness is 5-60m and the distribution area is 3.25×104km2. For Mud shale, those values are 3.60%, 5-125m, 5.11×104km2, respectively. For Silty shale, distributed discontinuously in the basin, the average TOC is less than 2.0% and thickness varies greatly.
The pore system of Source shale and Mud shale, with the TOC value greater than 2.0%, were studied. A series of new testing methods were used, such as FESEM, FIB-SEM, QEMSCAN, and quantitative analysis of pore, etc. The characters of pores were finely described, focusing on pore combination mode, pore shape, and pore scale. Three types of pore combination modes were summarized. The leading type is consisted of residual intergranular pore and dissolved pore in fine-grained clast matrix and intercrystalline pore in interstitial matter. Next one is consisted of interlamination pores and intergranular pore in organic matter. Last one is intragranular pore in organic particles.
The section of pores are irregular, strip and elliptical, with irregular the leading shape. The quantitative analysis results by Micro-CT and nano-CT show that multi-scale pores exist in Chang7 shale, from micro-scale pores to nano-scale pores. The radius scope of pores is 6-2000nm and the peak of distribution frequency arise when pore radius is 40-80nm. Nano-pores occupy the main position among multi-scale pores in Chang7 shale. Great difference can be seen between Source shale and Mud shale. For example, the scale of pores and the abundance of pores in two kinds of shale are quite different. That is caused by the content of terrigenous clastic particles. Because of high content of terrigenous clastic particles, Mud shale have rich residual intergranular pores and the scale of pore is larger.
It can be seen that fractures are abundant in Chang7 shale, tested by large scale methods, such as core observation, out crop measurement and by micro scale methods, such as cast thin section, SEM and CT. Most fractures exist with no filling materials. The areal fracture density in the shale measured from out crop is 3.09m-1. While, the data is larger than that of sandstone formation, interbedding with or close near the shale. The areal fracture density of the sandstone formation is 2.52m-1. Small scale analysis results show that fractures and micro-pores connect each other forming the storage and flow system for shale. Thus, in Chang7 shale, fractures act not only as efficient storage space, but also as flow path for fluid.
The geochemical analysis results show that, bitumen A of Chang7 Source shale is high(0.35-1.17%) and the content of saturated hydrocarbons in extration matter is also high(24.55-73.35%). While, bitumen A and content of saturated hydrocarbon of Mud shale is less than that of Source shale. Besides that, oil can commonly be seen bearing in newly drilled shale core. From all above discussed, Chang7 shale contains oil and gas in its special storage space.
Chang7 shale has good condition for fracturing. By whole rock analysis of X-ray diffraction, it can be seen that the content of brittle minerals in Chang7 shale, including quartz, feldspar, carbonate and pyrite, is very high with average value of 71.25%. On the basis of abundant natural fractures, complex fractures net can be easily created by using SRV, thus provide a good way for production of shale oil from matrix.
Pilot tests for exploration of unconventional shale oil in Chang7 Shale have been done in Ordos basin. Mixed water fracturing tests have been implemented in vertical wells in the favorable layers chosen according to features such as lithic facies, storage ability, oil bearing features and ability for fracturing. Commercial shale oil have been obtained in three wells. The continental liminc shale in China, taking Chang7 shale as the representative case, have potential for shale oil exploration. According to finely classification standard of Chang7 shale and the geological and geochemical characters of each type, Chang7 Mud shale is the main target for shale oil exploration in the near future.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013