Heterogeneous Physical and Mechanical Properties Relative to Composition, Porosity, and Diagenesis in Siliceous Mudstones of the Upper Monterey Formation, Belridge Oilfied, San Joaquin Basin, California

Abstract

Highly siliceous mudstones of the upper Monterey Formation exhibit a complex range of physical and mechanical properties at a fine spatial resolution. Unlike argillaceous mudstones, highly siliceous mudstones undergo a stepwise diagenetic transformation from opal-A to opal-CT to quartz phase silica with dramatic reductions in porosity and concurrent increases in brittleness. Increased clay-rich detritus in the initial sediment also reduces total porosity but with an associated decrease in brittleness. Prior studies of mechanical stratigraphy in the Monterey Formation focused on the heterogeneity of natural fracture patterns in outcrop. This study this study examines 40 cores from different burial depths to quantify the relationship between composition, porosity, diagenetic stage and hardness in subsurface rocks. XRF scanning and Leeb hardness testing (HLD) were performed at a 1 cm resolution. Hardness by a Proceq Bambino rebound hammer, is used to quantify mechanical heterogeneity. Results indicate that porosity is the principle influence on hardness while the HLD deviation from this trend is strongly influenced by silica-detritus ratios. The greatest shift in hardness occurs with the opal-A to opal-CT diagenetic transition (+55% HLD) where porosity reduction by fluid expulsion and matrix compaction was greatest. The conversion from opal-CT to quartz phase silica produced a smaller reduction in porosity and lesser shift in hardness (+12% HLD). Within opal-CT and quartz phase rocks, silica-detritus ratios have the greatest influence on hardness properties. In quartz phase rocks, doubling burial depth significantly increases hardness (+20% HLD) without further silica diagenesis. Data suggests that hardness increase and porosity loss in the deeply buried quartz-phase rocks was accomplished by silica enrichment via pore-filling cementation. These data can be used to model the mechanical stratigraphy of thin-bedded mudrocks. For example, in a diagenetically stratified opal-A to opal-CT transition zone, high porosity opal-A strata intercalated with highly-fracturable opal-CT strata may be exploited as a play with great mixed reservoir potential. The study also reveals the distinct properties of the very deeply buried siliceous rocks rarely encountered in outcrop or conventional drilling in California. Our findings emphasize that the processes responsible for porosity reduction are critical to understanding the strength and brittleness of siliceous mudstones.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017