Reservoir Micro Structure of Jurassic Da'anzhai Member Shelly Limestone and Their Hydrocarbon Significance in Central Sichuan Basin, China

Abstract

Tight oil of Daanzhai Member of Jurassic in Sichuan basin possesses 3 characteristics: the lowest porosity among all tight oil, the longest production history (nearly 60 years), and successful produc- ing tight oil without “horizontal well & hydraulic fracturing”. The article studied these characteris- tics through the reservoir micro structure, and 2 series of methods were used. One is qualitative tech- nologies including different optical microscopes and SEM, with which different scales of reservoir micro structure features were studied. The other one is quantitative technologies including high pressure mercury penetration, Nano-CT, and nitrogen adsorption, with which sizes and shapes of storage spaces can be characterized quantitatively. These two methods combined with production data, three topics were researched, including the types of reservoir rock and storage spaces, micro structure of storage spaces and its petroleum significance. By the study, three points were figured out. (1) Shelly limestone of Daanzhai Member can be divided into 7 types, while storage spaces can be divided into 4 types and 13 sub-types. (2) Most storage spaces are smaller than 1µm. Shape of 3 main types of storage spaces are similar with fracture, while as a whole, all storage spaces possess the characteristic of slit type. Almost each pore connects with 2-4 throats. Based on the micro res- earch, micron scale and submicron - nanometer scale of storage space structure models were built up. (3) The tiny size of storage space caused the low abundance of tight oil in study area, leading to the low production. Dual porosity system of pore & fracture is the main reason that why shelly lime- stone can maintain the production for a long period. Slit type of storage space shape together with multi-scale fracture system reduce the capillary force greatly, which enable tight oil be developed effectively in study area without “horizontal well & hydraulic fracturing”.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018