--> --> Compositional and diagenetic relationships and controls of mechanical behavior in siliceous mudstones of the upper Monterey Formation, Belridge Oilfield, San Joaquin Basin, California.

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Compositional and diagenetic relationships and controls of mechanical behavior in siliceous mudstones of the upper Monterey Formation, Belridge Oilfield, San Joaquin Basin, California.

Abstract

In the San Joaquin Basin, siliceous mudstones of upper the Monterey Formation (UMF), including the Belridge Diatomite, Reef Ridge, Antelope Shale, and McDonald members, exhibit a complex range of physical and mechanical heterogeneity with greater variation than would be predicted from bulk silica diagenesis alone. The UMF has a primary composition of biogenic silica and clay-rich detritus with minor calcite, ash, sand, and other components that are intermixed to thinly interbedded. With burial, biogenic silica undergoes a stepwise diagenetic transformation from opal-A to opal-CT to quartz-phase silica. Each transformation results in steps of decreased porosity and increased brittleness that may be gradual or abrupt depending on the ratio of silica: detritus (Si:Al). However, within any single diagenetic phase, abundant clay-rich detritus also reduces porosity and yet decreases brittleness. Consequently, mechanical definition from standard wireline log suites is difficult due to these conflicting influences on trends of porosity and brittleness. Previous outcrop studies have clearly shown relationships between fracture length, fracture style, bedding thickness, and lithology, but lack a direct quantification of mechanical properties. In this study, we use Leeb hardness, defined as the resistance to permanent deformation, and geochemistry to quantify variation in more than 120 feet of UMF diatomite, porcelanite, chert, and siliceous mudstone core from the Belridge field, San Joaquin Basin, CA. Lithologic variation is documented by physical examination and XRF-derived elemental ratios at 1 cm (0.4′) increments. Si:Al ratios, used as a proxy for silica vs. detrital content, vary over an order of magnitude from 10 to 100. Leeb hardness, measured by a Proceq Bambino rebound hammer, ranges from 320 to 780 HLd. Preliminary results show a large step of diagenetic hardening from opal-A to opal-CT lithologies, and that already quartz-phase rocks continue to harden with continued burial. Clay-rich lithologies have lower hardness than clay-poor rocks in every diagenetic phase. These preliminary results suggest that although bulk silica diagenetic phase has considerable impact on mechanical properties, varied sediment composition between discrete beds also have a significant influence on the fracturability of UMF lithologies. Understanding trends and thresholds of mechanical heterogeneity vs. composition and diagenetic phase can be directly applied to predicting wellbore stability, predicting proppant embedment, and/or targeting zones of potentially high fracture permeability.