Map-Based Volumetric Calculations of Probabilistic OOIP for Stacked Reservoirs with Multiple Oil Water Contacts: Examples from the N'Sano Pinda & Takula Lower Pinda Reservoirs, Block 0, Angola

Petro Papazis, Jerome Glass, and Antonio Ingles
Chevron International Exploration & Production, Houston, TX

Deterministic map-based volumetric techniques have historically been used as the approach for estimating Original Oil-In-Place (OOIP). The use of probabilistic methods has grown in application, and provides OOIP distributions which are linked to the uncertainties associated with input parameters. Generating these distributions are especially challenging in stacked reservoir systems with multiple oil water contacts. These reservoirs have uncertainties that are shared between the entire system together with uncertainties specific to individual units. A process is described to evaluate map-based probabilistic OOIP using Monte Carlo simulation in conjunction with Experimental Design (ED).
Monte Carlo simulation is used to combine the individual stacked reservoir OOIP distributions into a single total-field distribution. The stacked reservoir OOIP distributions are derived using both ED and Gocad's EVolUM. OOIP is a function of both gross-rock-volume (GRV) under closure and hydrocarbon pore volume (HCPV). ED is used to generate a single shared HCPV distribution for the entire stacked system from four parameters including permeability cutoffs, and the log transforms for permeability, porosity, and water saturation. ED is required to generate a proxy equation that describes the relationship between OOIP and the uncertainties, since no analytic equation exists that can be used in MC simulation. EVolUM is used to calculate reservoir specific GRV distributions attributed to structural and oil water contact depth uncertainties.
Two Albian-age reservoirs from offshore Angola, Block 0 are used to illustrate this technique. Both the N'Sano Pinda and Takula Lower Pinda reservoirs are mature stacked oil reservoirs and have recovered 50 MM and 40 MM barrels respectively since production began. The shallow marine, mixed clastic and carbonate sandstone Pinda Formation is the primary producing interval for both reservoirs.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece