Abstract

A3-D geological model was constructed from a 3-D outcrop for reservoir flow simulation that can address the effects of small-scale (subseismic) interwell heterogeneities on potential hydrocarbon production in analog deep-water oil and gas reservoirs. Dimensions of the Hollywood Quarry are 380 × 250 × 25 m (1247 × 821 × 83 ft). The quarry exposes the upper Jackfork Group turbidites in three dimensions. The Jackfork is often used as an outcrop analog for deep-water reservoirs in the Gulf of Mexico and elsewhere. A variety of pebbly and sandy turbidite facies are folded, cut by faults and fractures, and separated by laterally continuous shales.

Techniques used to characterize the quarry include photomosaic mapping, behind-outcrop coring, outcrop gamma-ray (GR) logging, measured stratigraphic sections, sequential photography of quarry walls, digital orthophoto-quadrangle (DOQ) mapping, ground penetrating radar (GPR), global positioning system (GPS) mapping, shallow high-resolution seismic reflection (SHRS), and GPS laser-gun positioning of geologic features in 3-D space.

The west wall has been quarried back to within 0.5 m (1.6 ft) of the first inline of a previous 3-D GPR survey and coring operation. The strata imaged along inline 1 are now exposed at the quarry wall. Core and photomosaic descriptions superimposed on inline 1 show good bed correlations and reveal small faults. Similar features are observed on seismic reflection traces. SHRS and a long conventional core provide details of the geology beneath the quarry floor. These details can be correlated updip to the east quarry face.

A GoCad™ 3-D geological model was constructed from the data. The model includes spatially oriented stratigraphic and structural features, and various upscaling combinations for evaluating the constraints that a coarser scale grid places upon the results of reservoir flow simulation. More than 100 such combinations were simulated.

The following reservoir features are found in the Hollywood Quarry: (1) fractures, (2) asymmetric anticline, (3) complex stratigraphy with massive sandstones, a shale barrier, and layered sandstones and shales, and (4) faults (with gouge zones). Flow simulations for features 2, 3, and 4 were performed. Each case was created by a combination of these features and several variables such as reservoir drive mechanism, fluid-flow characteristics, and well orientations.

Results indicated that the presence or absence of faults is a major factor when producing this analog reservoir. Simulated production decreases 0.1–15.0% when faults are incorporated into the simplest tank model. Partially sealing faults produce a decrease in production of 1.0 to 10.0%. A horizontal well across faults provides the best production results with an increase of 4.0 to 21.0% compared to a vertical well.

Very simplistic models, such as the tank model, increase OOIP (original-oil-in-place) estimates by 17.0% and oil production by 12.0%, compared to more geologically realistic reservoir models. Adding geological information to the model, such as shale boundaries and faults, increases the accuracy of the initial volumetric calculations.

The 0.1 km² (24 acres) of geology exposed at the Hollywood Quarry are representative of common interwell spacing present at many onshore oil and gas fields. Three stratigraphic facies, at least 15 faults, and various sets of fractures have resulted in a highly compartmentalized, both horizontally and vertically, stratigraphic interval. The Hollywood Quarry is a very useful area for understanding reservoir problems, including very close spacing and interwell complexity, log-shape-uncertainty interpretation and correlation, 2-D and 3-D seismic resolution, and potential field-production problems.