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Cementation in Los Angeles Basin: Compositional and Isotopic Constraints

Richard L. Coffman

Evidence of carbonate and laumontite cementation has been documented in wells from several oil fields within the Los Angeles basin. Petrographic examination reveals that carbonate cementation can be both early and late diagenetic phases. Early calcite commonly replaces the matrix but preserves the detrital grains, whereas later carbonate cement may replace matrix and fill porosity resulting from earlier dissolution. Laumontite cement is commonly observed replacing the matrix and detrital components (primarily plagioclase). Compositional variations (particularly trace elements) occur within carbonate cements and are useful in determining the relative timing of multiple cementation events, whereas the composition of laumontite cement seldom varies from its stoichiometric fo mula.

Oxygen isotope analyses of the cements permit limits to be placed on the temperatures of crystallization, which also places further constraints on the timing of cementation. Strontium isotopes are also being used as a tracer to determine the potential sources of calcium necessary to crystallize both calcite and laumontite. Comparison of strontium isotope ratios (87Sr/86Sr) from calcite and laumontite cements reveal whether the source of calcium is the same for both cements or different. The determination of possible calcium sources together with the timing of cementation enable calculations to be made concerning the timing and amount of mass transfer that has occurred.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.