Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Oil And Dolomite In The Monterey Formation Of California


Dolomite has a unique relationship with hydrocarbons in the Monterey Formation and related deposits of California, both as fractured or matrix reservoir and in the very origin of the mineral. Along with silica, kerogen, phosphate and calcite, dolomite is a key non-clastic component of the Monterey. Yet little is known about the paragenesis of petroleum and dolomite. Nor is much understood about the reason for large intra- and interbasinal differences in abundance. Dolomite generally forms distinct nodules, beds, laminations, veins, and more abundant, but less distinctive dolomitic lithologies (mudstone, diatomite, porcelanite, chert). Most stratigraphic dolomite is authigenic — forming at horizons in organic-rich sediments where pore-water iron, nitrate or sulfate is reduced and bicarbonate or methane is abundant. Nodular dolomite frequently formed around reducing, organic nuclei, such as bones or driftwood or in reactant-limited environments. In many successions, dolostone horizons are remarkably rhythmic in spacing, likely reflecting pauses in sedimentation related to relative sea-level cycles that form parasequences. Irregular, discontinuous dolostone beds are also commonly associated with slumps or other synsedimentary remobilizations that enhanced vertical permeability by disrupting continuous primary lamination. Initially disseminated or laminations of rhombs in siliceous rocks recrystallize and grow in conjunction with the opal-CT to quartz silica phase transition. Additionally, dolomite forms veins, fracture-fillings, and breccias that are either bed-confined or associated with cross-cutting fault zones. Oil and dolomite are spatially related in a number of ways. Fracture aperture and permeabilty tends to be high in dolostones because the greater thickness of mechanical strata than for most other lithologies in the Monterey Formation. Coarsely sucrosic dolomite is commonly composed of dolomite rhombs with nuclei of petroleum, suggesting that hydrocarbon seepage was related to dolomite formation in some locations. The most obviously petroliferous intervals of many cores are dolomites with highly saturated intercrystalline porosity. And oil is also found in cemented dolomite breccias as fluid inclusions, vugs, and as mm- to cm-scale bubble-like spheres surrounded by radiating bladed dolomite crystals in fault zones.