--> How Hard? How Porous? How Permeable? Compositional and Diagenetic Control of Physical Properties in Highly Siliceous Sedimentary Rocks of the Miocene Monterey Formation, California

AAPG ACE 2018

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How Hard? How Porous? How Permeable? Compositional and Diagenetic Control of Physical Properties in Highly Siliceous Sedimentary Rocks of the Miocene Monterey Formation, California

Abstract

Unlike clastic-dominated mudstones, highly biosiliceous mudrocks of the Miocene Monterey Formation of California undergo dramatic and abrupt changes in physical properties key to petroleum reservoir performance. These sediments show a complex yet predictable relationship between the variations in composition (biosilica/detritus ratio), diagenetic state, and burial history and their resulting hardness, porosity, pore structure and permeability. Original biosilica-rich sediment is composed largely of diatoms, radiolarians or sponge spicules that consist of hydrous silica termed opal-A. Highly diatomaceous sediments have an enormous amount of intra- and interparticle porosity, commonly 55-80%. With increased time or temperature, or in the presence of localized geochemical gradients, this material undergoes a two-step transition to diagenetic opal-CT then quartz. Porosity is generally reduced with each transition (typically to 20-45%, then 10-30%), although significant secondary porosity can be generated under certain burial histories. Steps in permeability between rocks of different silica phase is not correlated with % porosity because of surprising, but understandable changes in the shape, size and connectivity of pores.

Measurements of Leeb-hardness and composition by X-ray Fluorescence at >2000 locations on cores from the Belridge oil field area show important relationships between silica content, silica phase and hardness. XRF data from core scans were converted into % biogenic/diagenetic silica and % detrital aluminosilicate minerals on a carbonate-, phosphate- and pyrite-free basis. For almost any individual silica:detritus ratio, opal-A, mixed opal-A+CT, opal-CT, and quartz phase rocks show marked steps of increasing hardness with degree of diagenesis. Surprisingly, opal-CT phase rocks at ~2200’ and those buried to 3600’ showed no difference in porosity or hardness, likely reflecting resistance to compaction by the rigid crystalline framework. In contrast to transitions between other silica phases in rocks of most compositional ranges, only the opal-CT to quartz transition for rocks with >80% silica showed no increase in hardness at the diagenetic transition – again likely due to the strength of a rigid crystalline framework. Within any single diagenetic phase, samples also demonstrate a positive linear relationship between hardness and silica:detritus ratio, except for the limited opal-A dataset.