Nanoscale Pore Structure Characteristics of Lower Ganchaigou Formation Shale Reservoir in Western Qaidam Basin, China
Yuan Yuan1,2, Zhenxue Jiang1,2, Peng Li1,2, He Bi1,2, and Yucheng Wen1,2
1State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China
2Unconventional Natural Gas Institute, China University of Petroleum, Beijing, China
Qaidam Basin, located in the northern region of Tibetan Plateau, is the Mesozoic-Cenozoic intracontinental composite sedimentary basin, like an irregular diamond with NWW direction. Western Qaidam Basin is the main oil and gas exploration area in the Basin.
We took lower Ganchaigou Formation of western Qaidam Basin for example. We did the nitrogen adsorption test to study nanoscale pore structure of shale reservoir. Through analysis of adsorption isotherm and hysteresis loop as well as pore diameter and specific surface areas, we characterized the pore structure of shale reservoir and discussed how mineral component and total organic carbon (TOC) dominate the nanoscale pore structure and how significant to shale gas accumulation.
Experimental results reveal that the Lower Ganchaigou Formation shale reservoir has very complex pore structure which is rich in nanoscale pores. The type of this adsorption isotherm is similar with BET type II adsorption isotherm. The second half of the curve (p/po=0.8~1.0) presents phenomenonhat sharp increase and none adsorption saturation, which means capillary condensation happened. Most of samples get the hysteresis loops, which means that pore morpholohy of shale reservoir are mostly open. The shape of hysteresis loop is similar with type H3 loop from IUPAC classification, which means that the pores morphology are mostly ruleless and pore structure is mainly composed of slit-shaped pores paralleling to pore wall and many other pores with various forms as well.
The analysis of pore diameter and specific surface areas exhibits that mesopore (d=2~50nm) occupies most of space among nanoscale pores, with a mean proportion of 69.663%. Macropore (d>50nm) and micropore (d<2nm) respectively occupy 27.384% and 3.033%. Pore specific surface area is also mainly contributed by mesopore, with content of 84.650%. Furthermore, the one with less than 10nm pore diameter contributes most to the total specific surface area. In addition, the specific surface area of micropore and macropore respectively occupy 12.165% and 3.354%.
The mineral components of Lower Ganchaigou Formation shale mainly are clay minerals and brittle minerals which comprise quartz, feldspar, calcite, etc. We correlated pore volume and pore specific surface area with minerals content and TOC, and the result showed pore volume and specific surface have certain correlations with clay minerals and no correlation with brittle minerals and TOC. Some factors are main cause for these relationship. These phenomena generate because of some parameters, such as mesopore which is the primary nanoscale pore type in shale reservoir with most of specific surface area, clay minerals which have relatively more nanoscale pore and specific surface area, brittle minerals which distribute to larger pore, relatively low TOC and low thermal evolution extent.
In conclusion, Lower Ganchaigou Formation shale reservoir in western Qaidam Basin is rich in nanoscale pores. The pore diameters are much smaller, which results in stronger adsorption capacity to shale gas. Therefore, shale gas has been discovered in this place. Moreover, the pore staying in open state assists in transferring the differential pressure and then increasing desorption efficiency for shale gas as well as improving the permeability of reservoir, so that shale gas would have good productivity.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013