ABSTRACT A robust set of modal composition data (238 samples) for Eocene to Pliocene sandstone from the Cook Inlet forearc basin of southern Alaska reveals strong temporal trends in composition, particularly in the abundance of volcanic lithic grains. Field and petrographic point-count data from the northwestern side of the basin indicate that the middle Eocene West Foreland Formation was strongly influenced by nearby volcanic activity. The middle Eocene to lower Miocene Hemlock Conglomerate and Oligocene to middle Miocene Tyonek Formation have a more mature quartzose composition with limited volcanic input. The middle to upper Miocene Beluga Formation includes abundant argillaceous sedimentary lithic grains and records an upward increase in volcanogenic material. The up-section increase in volcanic detritus continues into the upper Miocene to Pliocene Sterling Formation. These first-order observations are interpreted to primarily reflect the waxing and waning of nearby arc magmatism. Available U-Pb detrital zircon geochronologic data indicate a dramatic reduction in zircon abundance during the early Eocene, and again during the Oligocene to Miocene, suggesting the arc was nearly dormant during these intervals. The reduced arc flux may record events such as subduction of slab windows or material that resisted subduction. The earlier hiatus in volcanism began ca. 56 Ma and coincided with a widely accepted model of ridge subduction beneath south-central Alaska. The later hiatus (ca. 25–8 Ma) coincided with insertion of the leading edge of the Yakutat terrane beneath the North American continental margin, resulting in an Oligocene to Miocene episode of flat-slab subduction that extended farther to the southwest than the modern seismically imaged flat-slab region. The younger tectonic event coincided with development of some of the best petroleum reservoirs in Cook Inlet.

Abstract The Cook Inlet Basin is a northeast-trending collisional forearc basin that extends from Shelikof Strait northeastward to the east end of the Matanuska Valley. The basin is divided into three segments including, from northeast to southwest, the Matanuska Valley segment, Cook Inlet segment, and Cook-Shelikof segment. The matanuska Valley segment represents the collapsed onshore part of the basin. The Cook Inlet segment is a significant hydrocarbon province, with more than 1.3 billion barrels of oil and about 8 trillion cubic feet (TCF of gas produced since 1958. No commercial oil or gas production has been established in either the Cook-Shelikof or Matanuska segments of the basin.

ABSTRACT The depositional environments and reservoir characteristics of the upper Miocene Etchegoin and Chanac formations in Kern Front oil field, eastern San Joaquin basin, California, were interpreted in cores from three wells. Sedimentary facies were correlated to the log responses and then mapped throughout the reservoirs. The Etchegoin Formation is a shallow-marine unit consisting of a basal transgressive and overlying deltaic units. Facies include shoreface, river-mouth bar, prodelta, paralic, and marine and distributary channel deposits. The shoreface deposits trend north-south, and are interbedded with, and overlain by, bioturbated marine units. They are gradational upward into river-mouth bars and marine channels, and are incised locally by distributary channels. The Chanac Formation underlies the Etchegoin Formation and contains meandering stream sequences deposited on a low-relief, mud-rich coastal plain. West-trending channels are recognized in the Chanac Formation. Log-derived data, combined with core porosity and permeability measurements, indicate that the upper Miocene reservoir sandstones have an average porosity 36.5% and average permeability 2,200 md. The Etchegoin sandstones are poorly indurated, arkosic arenites and wackes composed of subangular, medium- to coarse-grained, poorly-sorted detritus. Detrital modes for the sandstones are Q33 F47 L20 and Qm38 P48 K14. Authigenic minerals are rare and include calcite, illite, and Ca-zeolite. The best reservoir sandstones in the field are the Etchegoin deltaic shoreface and channel facies and the Chanac channel facies. The two factors controlling reservoir quality are grain size and sorting. The proportion of detrital matrix ranges from 11 to 12% in the better reservoir facies to 22 to 45% in the poorer reservoirs; the proportion of the coarsest-grained fraction (sand/gravel) ranges from 54% in the poorer reservoirs to 83% in the better reservoir facies.

Abstract 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 Q 37 F 54 L 9 , and the average partial mode including only the monocrystalline mineral grains is Qm 39 P 40 K 21 . 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 (R O ), 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% R O corresponding to a paleotemperature of 110°C (4572 m burial depth), with complete albitization first occurring at a reflectance of 0.90% R O 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% R 0 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% R 0 and a paleotemperature of 110°C. In this well, incipient albitization begins at 0.35% R O (77°C), with complete albitization occurring at roughly the same burial depth (2515 m) and reflectance (0.5% R O ) 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.