ABSTRACT: A New Geological Reservoir Model for the Monterey: An Alternative to the "Fractured Shale" Model

GOTER, EDWIN R., MARK G. PICHA, MELISSA A. SANDSTROM, and DAN E. SCHWARTZ, Shell Western E&P Inc., Houston, TX

The Monterey Formation is a fine-grained, siliceous basinal deposit that is the primary reservoir in major oil fields in the Santa Maria and San Joaquin basins and the Santa Barbara Channel of California. A widely held model depicts Monterey reservoirs as "fractured shales" wherein effective porosity, permeability, and storage reside solely in fractures. However, the long-term production performance of many Monterey fields is atypical for "normal" purely fracture reservoirs. We here present evidence to support a new dual-porosity reservoir model as an alternative which better describes quartz-phase Monterey reservoirs and their production performance.

The proposed model resulted from multidisciplinary studies conducted at Shell in the 1980s. It depicts a dual-porosity reservoir wherein effective fracture and matrix porosity coexist in "clean" quartz-phase siliceous rocks below the thermally induced opal-CT-to-quartz

diagenetic boundary. Key data supporting the model were derived by extensive SEM, TEM, and petrographical studies linked with petrophysical analyses using cores and modern log suites. The Monterey was classified into five compositionally and texturally distinct rock types: (1) "porous chert," (2) "porous dolomite," (3) "glassy chert," (4) "tight dolomite," and (5) "shale."

A significant proportion of the most siliceous Monterey sequences consist of "porous chert," a microcrystalline, very clean (i.e., nonshaly) quartzose rock composed of 0.5-2u crystals. Porous cherts are excellent, albeit low permeability, reservoirs that contain 20-35% effective matrix porosity with capillary entry pressures of 100-500 psi Hg. Reservoir quality is extremely sensitive to the clay content of the rock, with small amounts adversely affecting permeability and effective porosity. Porous cherts are widely distributed. They have resulted from deposition of relatively pure diatomaceous ooze, undiluted by terrigenous clay sedimentation, which subsequently was converted to quartz phase via the progressive stages of silica thermal diagenesis. Porous dolomites commonly have better reservoir properties than porous cherts but are viewed overall as secondary in importance. The shale rock type represents a wide range of terrigenous-dominated, fine-grained rocks that have little usable matrix porosity. The other two rock types contain usable porosity only where fractured.

Although thinly bedded, the Monterey commonly contains thick intervals dominated by a particular rock type that can be differentiated using modern log suites. Resistivities of porous chert sequences vary significantly as a function of fluid content as well as lithology. This allows rough water saturation estimates to be made using appropriate techniques with sufficient core, log, and production calibration.

This alternative model predicts that in traps with sufficient column height, entry pressures of porous cherts would have been overcome and the high matrix porosity saturated by oil. Hence, BOIP would be much greater than in fracture reservoirs with typical usable porosities of less than 1%. In this model, oil moves to the wellbore through fractures present in both porous and nonporous rock. Fracture production is augmented by matrix contribution. A major uncertainty is the matrix recovery factor. Recovery efficiencies should vary widely because of factors such as shaliness of the porous cherts and fracture density. As a result, reservoir modeling studies of recoverable reserves give mixed results. Even in favorable cases, recovery factors are small. However, the large volume of matrix storage defined results in significantly better long-term well performance than for purely "fractured shale" reservoirs and correspondingly, better EURs.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)