Determination Of Fracture Intensity And Reservoir Potential In The Monterey Formation, Point Arguello Field, Offshore California

The Point Arguello Field structure is a doubly plunging anticline which is complexly fractured as a result of horizontal compressive stresses. The main reservoir is the Miocene Monterey Formation which is composed of interbedded mudstones, porcellanites, cherts and dolostones. Although the Monterey Formation lithologies contain minor primary porosity, the predominant, porosity-permeability system is due to intense fracturing. The magnitude and style of fracturing are influenced by lithology. The more plastic and massive mudstones have isolated, through-going fractures; the more siliceous rock types such as brittle cherts are highly brecciated and rubblized. Traditional log interpretation does not adequately represent variations in reservoir potential due to the intense fracturing of interlaminated lithologies. All available modern logging devices (as of 1987–88), with the exception of a borehole televiewer, were run in a series of development wells from the Hermosa and Hidalgo Platforms. The most useful tool for selection of perforation intervals was the full waveform sonic log. Acoustic waveform patterns correlated very well with various degrees of fracture intensity recognized in 692 feet of recovered core from the OCS-P-0316 11-2 well drilled from the Hermosa Platform. While the compressional and shear wave velocities both vary according to lithology, the shear wave amplitudes showed broader variations than the compressional wave amplitudes, corresponding to varying degrees of fracture intensity. A normalized computation of shear wave amplitudes, taken from the closest receiver, provide a qualitative method of modelling fracture intensity. Four levels of fracture intensity identified from core were assigned to equal quartiles of the normalized shear amplitude computation: 0–25 (0.1 - 1.0 cm fracture spacing); 25–50 (1–4 cm fracture spacing); 50–75 (4–10 cm fracture spacing); and 75–100 (10–100 cm fracture spacing). This technique has been used to correlate fractured zones from well to well across the field. Initial production logs run in the cored well and other wells, indicate a good correlation between amount of fluid entry from perforated intervals and the variation in shear wave amplitudes. In intervals where this correlation is not as good, formation damage is interpreted, and stimulations will be attempted.