The Okhotsk Sea Shelf Siliceous Horizon and Sand Reservoir Identification Criteria Based on the Well and Seismic Data

Abstract

Results of the four exploration wells drilling on the Okhotsk Sea shelf in 2006-2008 had discovered no terrigenous reservoirs in those sections where the sand packages were expected based on the seismic data.The penetrated lithological stratigraphic complexes feature the siliceous rocks of the biogenic genesis in the formations. In course of compaction, dehydration and diagenesis the rock properties change with the depth. Silicon modifies from opal through christobalite into chalcedony (quartz) while the diatom ooze converts into diatomites, gaizes and silica rocks. The physical properties such as porosity, density, P-wave velocity and some other get similar to same of the sand rocks. This frequently results in the wrong forecasting of the reservoir seismic and petrofacies and distinguishing the so called "pseudoreservoirs".Study of soils and core sampled from the siliceous sediments penetrated by the wells drilled in the Okhotsk Sea area detected an abnormally low density of the diatomite sediments skeleton varying as function of the diagenensis from 2.15 g/cm3, 2.38 g/cm3 and 2.65 g/cm3 and high subcapillary porosity.For reference, the sand reservoir densities in the Okhotsk Sea area vary from 1,9 to 2,55 g/cm 3 .At present a confident distinguishing between the sand reservoirs and "pseudoreservoirs" in a significant number of the intervals delineated by the 3D seismic survey is only possible after drilling a well with a subsequent full range of well logging and DST and formation and core sampling.The siliceous intervals may have all reservoir features based on the well logging data, resistivity penetration into the different depth curves and mud cake based on the caliper logs, negative SP anomalies and low radioactivity. Therefore determination of the siliceous intervals based on the well logging data before the testing is critical. Below follow the results of the well studies which showed their prospectivity for identification of the siliceous intervals in formation.Analysis of the spectral gamma log curves (Fig.1) had shown that the reduced content of thorium in its turn being a result of the reduced terrigenous particles content is a reliable criterion for finding siliceous intervals in a well.