Secondary Porosity and Reservoir Quality of Lower Tertiary Sandstones in Dongying Depression, Eastern China

Xiaomin Zhu, Dakang Zhong, and Qin Zhang
University of Petroleum, Beijing, China, Beijing, China

A study of thin sections, scanning electron-micrographic data, and porosity-permeability measurements indicated that the porosity of lower Tertiary sandstones in Dongying Depression is mainly primary porosity in the shallow section (<1600 m), and predominantly secondary porosity in the deep section (>1900 m). The secondary porosity is originated from the dissolution of carbonate cement (up to 20%) and feldspar (about 3%). The secondary porosity in the shallow section was originated from calcite dissolution caused by organic acids during organic matter maturation; the later secondary porosity from Fe calcite dissolution was distributed below 2500 m and was related to the CO2 from decarboxylation of organic acids and the thermal cracking of organic matter. In the process, the dehydrating of clay promoted the dissolutions. The distribution of secondary porosity also varied spatially. In the northern part of the depression, the secondary porosity distributed from 1650 to 2450 m, and also probably from 2800 to 3500 m. In the central uplift, secondary porosity developed below the 1650 m, especially below 1900 m. In the south, secondary porosity was poorly developed. Sandstone reservoir quality is controlled by compaction, carbonate cementation/dissolution and lithofacies. Carbonate cementation has significant impact on reservoir quality. Where carbonate cement is less than 15%, sandstone porosity is commonly greater than 20%. While carbonate cement exceeds 15%, reservoir quality is much poorer. Carbonate dissolution can improve reservoir quality considerably. Regionally, dissolution was controlled by tectonics and depositional systems/burial history. Locally, sandstones lithology played an important role. Well-sorted, medium-fine grain sandstones with a low matrix content were subjected to heavy dissolution.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005