Challenges in Reservoir Characterization and Original Oil-in-Place Estimation in the Presence of Very Fine Bedding: From Example from a Mixed Clastic-Carbonate Reservoir from Block 0, Offshore Angola

Daniel M. Kiala, Antonio M. Ingles, Steve Jenkins, Sebastien Bombarde, Yahya Fitor, Maria Silva, and Julia Correia
Chevron Africa/Latin America Exploration & Production, Luanda, Angola

The Vuko Mesa reservoir is a finely laminated mixed clastic carbonate sequence deposited in a marine shelf environment. Reservoir quality varies rapidly vertically due to the influence of minor transgressive and regressive cycles. Mesa stratigraphic sequences are intercalated with the deposits of fine grained sandstone and silststone to very fine grained and shaly sediments with argillaceous markers. The best producing reservoir intervals are typically 1-2 ft thick in the Mesa reservoir.

The presence of very fine bedding poses a serious challenge to formation evaluation, and to the estimation of oil volumes and oil recovery. Reservoir units on the order of several inches thick are below the resolution of conventional wireline logs. This leads to difficulties in estimation of reservoir properties that are fundamental to oil recovery. Net to gross can be overestimated in this case, along with under-estimation of reservoir porosity and over-estimation of water saturation in the dynamic reservoir model. This problem has recently been addressed by a field cross-functional team of reservoir engineers and earth scientists working to integrate available static and dynamic data.

At the Vuko Mesa Field, core data was reviewed to determine a core-based net to gross ratio. Core data was used to develop new transforms of wireline porosity and permeability processing using the GEOLOG program. Enhanced Vertical Resolution (EVR) processing of the raw wireline field tapes was also conducted in an attempt to obtain net to gross ratios consistent with core based estimates. These data have been integrated into a new three dimensional reservoir model to improve waterflood management of Vuko field.

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