Identification of Lateral Facies Trends Using Seismic and Well Velocity Data, South Lokichar Basin Kenya

Abstract

Exploration drilling in the South Lokichar Basin has focused on targeting fluvial sandstone reservoirs (Auwerwer Fm) trapped within hangingwall anticlines to the west of the basin, with 10 hydrocarbon accumulations discovered between 2012 and 2017. Appraisal drilling has identified a lateral, east to west facies transition from distally sourced axial fluvial deposition towards the centre of the basin to proximally sourced alluvial fan deposition near the western basin bounding fault. Wireline log and core data indicate the alluvial fan facies is of very low porosity and is deemed to be non-reservoir. Accurate delineation of this alluvial to fluvial facies transition is an important consideration for both volumetric analysis and field development planning. Detailed field scale seismic interpretation and attribute analysis has been undertaken to try and delineate the fluvial to alluvial facies transition but it has not been possible to produce a reliable seismic amplitude based predictor of this facies transition. A relationship between depositional facies and velocity compaction gradient was identified from analysis of sonic logs and VSP data. The intervals dominated by the alluvial facies exhibit a larger compaction gradient than the intervals dominated by the fluvial facies. It is thought that different mineralogy, grain size and sorting is the primary cause of the velocity compaction gradient differences, with the alluvial facies being composed of more immature, poorly sorted and coarser grained material than the fluvial facies. Analysis of seismic pre-stack time migration velocities from one of the fields demonstrates a good correlation between calibrated well velocity logs and the seismic processing velocities, especially for a well which penetrated alluvial fan and exhibits an increased compaction gradient. Seismic processing velocities were subsequently used to generate velocity compaction gradient maps for different reservoir intervals. The velocity compaction gradient maps allow for a distribution of non-reservoir alluvial fan facies to be input to static geological models both for volumetric analysis as well as for subsequent field development planning.

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