269 / 2019-07-03 16:21:22

Characterization and controlling factors of karstic reserviors in lacustrine carbonates of in the Upper Fourth Member of Shahejie Formation (Es41) of Paleogene, Western Dongying Depression, China

Karstification; reservoir characteristic; controlling factors; Dongying Depression; Paleogene

Based on observations of cores, thin sections, well loggings and seismic data, we analyzed the karst type and characteristics, and controlling factors of porosity distribution. Lacustrine carbonates in Es41 underwent successively depositional karstification, local karstification, eogenetic karstification and buried karstification. During the depositional stage, lots of intergranular solution pore and mold pore formed due to unstable component dissolution by meteoric water. Irregular dissolution grooves and layered distribution dissolution pores occurred locally at the top surface of carbonates. At the end of Es41, due to tectonic uplift, carbonates partly exposed and eogenetic karst formed in a geological setting similar to peninsula or island setting, formed flank caves on the periphery and spongy-like horizons inland mainly within 25 m beneath the top surface of the Es41. In the burial stage, nonselective dissolution pores formed by organic acid. In general, facies-controlled is a distinguish feature of the karstifaction. During depositional and local karstification stage, selective dissolution by meteoric water mainly occurred in grainstone. In eogenetic stage, dissolution produced by mixing process mainly formed in grainstone and algal limestone within 25 m beneath the top surface of the Es41. During burial stage, organic acid entered intervals with high porosity and permeability and dissolved grains and cements. Overall, high-quality reservoirs mainly formed in grainstone and algal limestone intercals within 25 m beneath the top surface of the Es41.