Monterey Formation is one of the most productive petroleum-bearing formations in California with substantial proven petroleum reserves both onshore and offshore. A key question in the study of the Monterey Formation is the role of the matrix in terms of its storage of hydrocarbons and its potential contribution to the total ultimate recovery. This issue itself affects the selection of a conceptual model for reservoir studies, resource management, and recovery optimization.

We conducted a study of petrophysical information and performance data from a number of offshore Monterey fields and conditioned our observations with other evidence from published geological studies. All indications are that the most prolific members of the Monterey Formation offshore contributing to well productivity are the brecciated quartz chert intervals (tiger striped chert) with substantial fracture permeability and fracture capacity. The geological processes leading to the formation of the quartz member, geological history of the chert intervals, and study of outcrops indicate that tight chert sections are not serving as pressure support for the fracture as originally thought. As such, the contribution of matrix to recoverable reserves is considered trivial. An interpretative study of the database confirms the existence of a massive network of microfractures communicating with major fractures. Storage capacity of microfractures is a component of the overall recoverable reserves. We have observed similar signals in analysis of core data, well logs, and pressure transient tests data, which confirms our position on this matter.

Diagnostic plots for pressure-time relationship for highly brecciated intervals resemble that of hydraulic fractures intersecting wells. Dual porosity responses observed in certain cases are attributed to the role of microfractures supporting the major fractures.

Drive mechanisms in the offshore Monterey fields are primarily aquifer support and solution gas drive. Communication among wells through the fractures and the aquifer stabilization is seen from interference tests and static pressure measurements.

Water production in certain wells can be excessive because of rapid movement of the oil-water contact (OWC) within the highly permeable fractured intervals.

A bundle of straws draining a tank is proposed as a conceptual model for effective description of the Monterey fields. In this type of a system, for estimating cumulative water influx within the drainage area of individual wells, we have used the X-Plot technique. This methodology allows the application of tank type material balance for aquifer influx calculation.

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