Fracture Evaluation of Block P-0315, Point Arguello Field, Offshore California, Using Core, Outcrop, Seismic Data and Curved Space Analysis
M. J. Padgett, D. C. Nester, 1991. "Fracture Evaluation of Block P-0315, Point Arguello Field, Offshore California, Using Core, Outcrop, Seismic Data and Curved Space Analysis", The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management, Robert Sneider, Wulf Massell, Rob Mathis, Dennis Loren, Paul Wichmann
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The fracture system in the Miocene Monterey section at Point Arguello Field, was studied to find the relationship between fracture intensity and local tectonic structure. The analysis included five studies: seismic structural mapping, seismic amplitude, outcrop, core, and surface curvature. The goal was to develop a predictive and quantifying methodology for the exploitation and exploration of fractured reservoirs of the type found in the Point Arguello field.
A detailed structure map on the Monterey chert member was constructed from a 3D seismic survey on a Landmark workstation. The time structure map was depth converted with an accuracy of ±100 feet to the lithologic Monterey chert unit within existing well control. Seismic amplitude studies were performed to relate fracture intensity to a reduction in seismic amplitude across the field. An outcrop study was performed at Lion's Head fault to determine the relationship between the distance to a large fault and fracture density. A statistical evaluation of core analyses for 6 wells by Terra Tek (Salt Lake City) was performed to evaluate the variability of fracture intensity as a function of structure.
The surface curvature analysis consisted of treating each fault block as a 2 dimensional curved space (closed-compact 2D manifold in R3). At each point the distance to the boundary (faults) was determined as well as measures of the local curvature. The Gaussian curvature, ellipsoidal curvature and second derivative were used to characterize the curvature. The distances to faults and curvature measures were correlated to core analyses and to each other. Results are consistent with the outcrop and core studies, and indicate an ability to use the topology of a seismically derived structure map to predict fractured reservoir parameters.
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The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management
Bima Field, offshore northwest Java, is a sizeable reservoir containing reserves of approximately 700 MM bbls OOIP with a 50 BCF gas cap. At present only the northern 1/3 of the field is developed, with 7 platforms and 54 producing wells, of which 20 are horizontal. The field has multiple drive mechanisms and high viscosity oil (21 cp), resulting in rapid GOR and water-cut increase after 3 years of production. The high stakes (both reserves and facility investments) and the reservoir's complexities, make an effective reservoir management scheme critical. For this reason an integrated geological, geophysical and engineering description was carried out to provide a 3-D Reservoir Simulation Model to evaluate development options. Geologically, the Oligo-Miocene age Batu Raja Limestone was deposited on the Seribu Platform, a basement-controlled, fault- bounded structure. The Upper Batu Raja carbonate build-up is thickest on the structurally highest parts of the platform where the rock comprises a series of "cleaning upwards" cycles (muddy deposits overlain by progressively more grain-rich sediments). A Lower Miocene drop in sea-level caused subaerial exposure of much of the platform and leaching by meteoric fluids. This diagenetic event resulted in contrasts in the reservoir quality (porosity, permeability, fluid saturations) at various intervals of the Upper Batu Raja. Based on these dissimilarities, the reservoir was zoned into 6 model layers. Once zonation was established, well logs could be calibrated to whole and sidewall core. A dense grid of seismic data were used to map the Batu Raja structure. From these data, color seismic inversion sections were produced and calibrated to the well logs. The calibrated seismic data were then used to map the top of structure, the carbonate build-up's edges, the total thickness of the Upper Batu Raja (needed to control aquifer size in the model) and the thickness of the main pay zone (layers 1-3). Engineering reservoir description began with a detailed compilation of capillary pressure, relative permeability, production and DST data. The 3-D simulation model required special treatments, including varying the GOC depths to honor separate gas cap closures; making permeability pressure dependent in poorly-consolidated zones; and setting up horizontal well completion treatments. Results suggest that water injection into the oil rim and gas cap is an effective approach toward maximizing recoveries and minimizing gas cap resaturation. However, waterflood reserves are sensitive to injection timing. The synergistic approach of geological, engineering and geophysical input into the Bima reservoir study has had impact by delivering a reservoir management tool that can evaluate future development expansion and possible gas sales. The simulation model can also track fluid migration during the field's producing life. The geological/geophysical model led to an enhanced understanding of Batu Raja depositional and diagenetic processes that has potential in regional exploration strategies.