Diagenetic Mineralogy and Controls on Albitization and Laumontite Formation in Paleogene Arkoses, Santa Ynez Mountains, California
Kenneth P. Helmold, Peter C. van de Kamp, 1984. "Diagenetic Mineralogy and Controls on Albitization and Laumontite Formation in Paleogene Arkoses, Santa Ynez Mountains, California", Clastic Diagenesis, David A. McDonald, Ronald C. Surdam
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The stratigraphic and lateral distributions of authigenic minerals in feldspar-rich Paleogene sandstones of the Santa Ynez Mountains, California, are important in determining their reservoir quality. The sandstones were deposited in an east-west elongate basin during two regressive episodes. Deep-water turbidites were overlain by shallow-water traction deposits and eventually by continental fluvial deposits as the basin was progressively filled from the east. Modal analyses document a common provenance for all the Paleogene sandstones consisting primarily of acidic to intermediate plutonic rocks, with minor volcanic, metamorphic, and sedimentary components. The average detrital mode of 27 sandstones is Q37F54L9, and the average partial mode including only the monocrystalline mineral grains is Qm39P40K21.
Textural relationships and the stratigraphic distribution of diagenetic minerals delineate the paragenetic sequence: (1 ) syndepositional to very early pyrite; (2) early concretionary calcite cement; (3) incipient dissolution of detrital heavy minerals and feldspars; (4) clay pore linings and pore fillings; (5) formation of sphene and anatase; (6) incipient albitization of detrital plagioclase; (7) quartz, plagioclase, and K-feldspar overgrowths; (8) dissolution of feldspar creating secondary porosity; (9) local precipitation of pore-filling kaolinite; (10) laumontite cementation and replacement of plagioclase; (11) barite cementation and replacement of detrital grains; and (12) late-stage calcite replacement of detrital grains and earlier cements.
Organic metamorphism, as expressed by vitrinite reflectance (RO), provides a means to correlate mineral diagenesis in the sandstones with the thermal history of the Santa Ynez basin. In the eastern end of the basin (Wheeler Gorge) incipient albitization is first recognized at 0.5% RO corresponding to a paleotemperature of 110°C (4572 m burial depth), with complete albitization first occurring at a reflectance of 0.90% RO corresponding to a paleotemperature of 165°C (5425 m burial depth). The first occurrence of laumontite is in the turbidite beds of the basal Matilija Formation (5669 m burial depth) at approximately 1.0% R0 reflectance (173°C). Further to the west, at Point Conception (Gerber No. 1 well), the first occurrence of laumontite is at an estimated burial depth of only 2515 m, corresponding to approximately 0.5% R0and a paleotemperature of 110°C. In this well, incipient albitization begins at 0.35% RO (77°C), with complete albitization occurring at roughly the same burial depth (2515 m) and reflectance (0.5% RO) as the first occurrence of laumontite.
The top of the laumontite zone occurs at greater burial depths and paleotemperatures in the eastern portion of the Santa Ynez basin than in the west. Laumontite distribution appears to be controlled by porefluid chemistry and post-compaction permeability variations, which are responsible for creating differences in fluid pressure between petrologically similar sandstones. “Dynamic” overpressuring may have occurred in the turbidite facies of the Juncal and lower Matilija Formations, whereby pore fluids enriched in Na+ from the dewatering of smectite-rich shales permeated into the turbidite sandstones at a faster rate than they were expelled. Under these conditions, a continuous supply of Na+ would have been delivered to the sandstones to allow albitization of calcium-bearing plagioclase, which in turn supplied Ca+2 necessary for the formation of laumontite.
The authigenic minerals in the lower Paleogene sandstones of the Santa Ynez Mountains render them ineffective as reservoirs. Better reservoir prospects occur in the upper Paleogene and Neogene sandstones, particularly in the western part of the basin where they have not been subjected to deep burial, and secondary porosity is well developed.
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.