Technical Paper: Advanced Borehole Image Applications in Turbidite Reservoirs Drilled with Oil Based Mud a Case Study from Deep Offshore Angola

Society: SPWLA
Paper Number: 2003aa
Presentation Date: 2003


Developing an accurate reservoir model and an effective exploitation program for deepwater turbidite plays requires a clear understanding of the context of wellbores within the turbidite system. It also requires subdivision of the reservoir interval into petrophysical or sedimentary facies. Commonly these objectives are achieved by coring, however the high cost of such operations and the limited intervals covered make high-resolution wireline imaging an appealing alternative.

Borehole image logs provide high-resolution directional data sets, in addition to sedimentary fabrics and texture. The data derived from these images are typically used for lithofacies classification, palaeotransport analyses, fault and unconformity recognition. A common tendency in conventional log analysis is to underestimate the proportion of thinly bedded sands. The highresolution imaging tool enables us to detect these thin sand beds. Here we examine the prospects for extending these applications to the oil-base environment using images acquired by the recently introduced Oil-Base MicroImager tool.

We compared OBMI (Oil base mud) images with core (slab photos) in detail for deep-water formations (sand-shale sediments of low resistivity) of Angola. We examined how far these images can be successfully used for the advanced applications (thin bed and structural analysis). This case study demonstrates the quite good accuracy of thinly bedded sands identification with OBMI images when compared with core. Conventional open hole logs, otherwise, do not identify these thin beds of sands. The use of high resolution RXO from OBMI resulted in increase of net sand counts. The reservoir facies (deposited under a specific set of sedimentological conditions) are identified on the OBMI images with combination of lithology and OH log response.