Control of original sediment composition on mechanical strength and reservoir properties of rocks in the Monterey Formation of the Santa Maria basin, California

Abstract

I measured outcrop-sections and described cores from the Monterey Formation in the Santa Maria basin for Union Oil Company. It was common to see fractured-rock layers interlayered with non-fractured layers. Research showed that variations in original sediment composition ultimately determined the mechanical properties of these rocks. The first studies of rock mechanics came from the field of Engineering Geology, since weak rocks had caused repeated problems with construction projects. That effort led to development of a formal engineering classification of rock strength, based on Uniaxial Compressive Strength (UCS). UCS tests provide a formal definition for brittle versus ductile rocks. Rocks defined as brittle undergo little plastic deformation and will fail by fracture soon after reaching their elastic limit. Rocks that are ductile will continue to deform by plastic flow without fracturing, long after the elastic limit is reached. Classic end-members of the brittle to ductile rock-spectrum are seen in comparison of UCS tests of granite and rock salt cores. A granite core shows very little plastic deformation prior to fracturing, while a rock-salt core displays major plastic deformation and never does fracture. Extensive research has shown that the properties of rocks with high compressive strength include having low porosity, low clay content, low kerogen content, high mineral toughness where mineral toughness is a function or mineral hardness and crystal cleavage (consider halite versus quartz as an example), and a high degree of interlocking of crystals in the rock matrix. Rocks with the opposite properties have low compressive strength. Examples on the high side include granite, chert, and silica-cemented sands. Carbonates are lower because of low mineral toughness. Weak rocks include porous sands, evaporites, and mudstones. Any sequence of Monterey rocks in the Santa Maria basin will contain both strong and weak rocks interlayered with one other. During tectonic compression associated with growth of structural traps, stiff rocks like chert will only deform by fracturing, while weak rocks like mudstones will deform by plastic flow without fracturing. The result is a stratigraphic section containing both fractured and non-fractured rocks. Reservoir quality of the Monterey does vary from place to place, depending on the proportions of brittle and ductile rocks at each location. That variation ultimately depended on lateral changes in original sediment composition within the Santa Maria basin.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)