Effect evaluation of shale types on hydrocarbon potential using well logs and crossplot approach, Halewah oilfield, Sab'atayn Basin, Yemen

Abstract

Log-analysts typically distinguish three types of shale distribution in clastic reservoirs, that is, laminated, dispersed and structural distribution, which tend to influence the effective porosity and permeability of reservoirs. The Alif Member is made up of sandstone interbedded with some shale, with shale dominating on top and at the bottom. In sedimentary formations, the gamma ray log normally reflects the shale content. The gamma ray log and neutron-density porosity crossplot is used to estimate a reservoir's shale volume (Vsh), according to which the Alif Member is divided into three zones. In addition, shale types are recognized by neutron-density porosity crossplot, and the correlation coefficients of gamma ray log-based Vsh and neutron-density porosity-based Vsh are suggested to be reliable. The Vsh and shale types of the three zones are compared against the reservoir's potential to yield hydrocarbons. The major oil intervals mainly concentrate in shaly sand and sandstone zones of dispersed and laminated shale types. The third zone, the lowermost interval, contains pure shale of laminated type and is considered out of reservoir standards in hydrocarbon production due to its excessive shale volume. Quantitative assessment of shale type distribution and Vsh by crossplot approach could not furnish correct information to evaluate massive shale intervals.