Diagenetic laumontite has been identified in samples of Cenozoic clastic rocks from 25 wells drilled in the southern end of the San Joaquin Valley. These occurrences, and data from 50 other wells that encountered no laumontite, define a sharp interface between zeolitized strata and overlying less altered beds. Contours on the interface have been mapped in the depth range between 2.1 and 6.1 km for an area of 2,300 sq km extending from the Bakersfield arch south to the San Emigdio Mountains. Companion maps depict generalized structure at the base of the Pliocene sedimentary section and at the basement floor.

Generalizations derived from data of other regions indicate that pore-filling laumontite crystallizes from interstitial water of mineralogically immature sandstone when (1) geothermal gradients equal or exceed the range set by 59°C at 1,100 m to 180°C at 4,150 m; (2) fluid pressure gradients are near 113 bar/km (0.5 psi per ft); and (3) the solutions have exceptionally low salinities and are depleted in dissolved carbonate species. Laumontite in the map area is mostly a product of conditions prevailing during Miocene to Pleistocene time, and is thus a relic.

Laumontite pore-filling in petroleum reservoir sandstones lowers porosity, drastically reduces permeability, and seriously limits the possibility of commercial production. Laumontite-bearing rocks are poor prospects for petroleum production unless there is evidence that mineralogically mature sandstone may be abundantly interbedded or that pore-fluid composition may be locally conducive to high carbonate activity.

The discordant diagenetic boundary in the map area is inferred to be the product of both lateral variation in geothermal gradients during crystallization and post-diagenetic faults and folds. Entrapment of petroleum may occur where a sloping diagenetic front steeply crosses gently plunging folds. Hydrocarbon traps that are in part diagenetic have been discovered mainly by accident. Other possible diagenetic entrapment geometries that are yet to be tested and are not associated with structural closures can be delineated from our maps.

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