Abstract The SEPM core workshop on deep-water clastic sediments was organized to provide participants with an opportunity to view cores from a variety of deep water depositional settings and to demonstrate the application of process sedimentology in the interpretation of depositional enviroments from the study of cores and associated subsurface data. The studies assembled for presentation in the workshop have dealt with sedimentary sequences which have been interpreted as having formed by deposition of non-calcareous, clastic sediment in relatively deep water (generally slope basins and greater depths). These studies also have been concerned principally with coarser deep-water sediments (usually fine-grained sandstone or coarser) of such stratigraphic sequences because of their potential as hydrocarbon reservoirs with primary and (or) diagenetically modified inter-granular porosity and permeability. Obviously those coarser parts of such deep-water sequences are anomalous in that they represent transport and deposition by processes that did not operate most of the time in the overall relatively quiet depositional settings. The probable processes of transport and deposition of such anomalous coarse clastic sediment, and overall models for dispersion and accumulation of such sediment, therefore have been considered in some detail in the studies included in the core workshop. However, this is not a course on sedimentary mechanics; it is a course in comparative stratigraphic and sedimentologic analysis. Six core sequences, which have been the subject of detailed sedimentological study, will be on display during the core Workshop. Two studies pertain to deep-water, Upper Cretaceous sandstones (mainly the Winters Sandstone) of the Sacramento Valley

Abstract The detailed sedimentological analysis of whole-diameter cores is greatly enhanced by the core being in good condition; ideally, cores should be complete (100% recovery) and well labeled with all pieces in correct order. Considerable information loss occurs when cores are mishandled; therefore, specific procedures should be followed. At the well site care should be exercised to preserve correct orientation of individual core segments and the core should be properly marked. Hügel orientation grooves cut in the core will facilitate core reconstruction. Commercial core analysis laboratories usually generate the greatest amount of information loss; therefore, special care should be used during subsampling for fluid-saturation analyses and porosity-permeability measurements. A core gamma-ray scan should be obtained whenever possible to aid in later core-to-log correlation. Finally, prior to detailed examination, it is recommended that the core be slabbed, lapped and photographed. X-ray radiography may be needed to enhance subtle sedimentary structures in massive-appearing sandstones and mudrocks. For proper sedimentological core analysis we recommend a process-sedimentology approach which emphasizes detailed lithologic description and the recognition of genetic units within the vertical sequence. A continuous, detailed sketch should be made and a description made using a check list of important lithologic features. An understanding of Walther’s Law is required for maximum use of the vertical sequence. At least four types of genetic units can be delineated to interprete the physical, biological and chemical processes responsible for generating the sedimentary rock product of the core. Those types are: sedimentation unit, ichnogenetic unit, soft-sediment-deformation (s-s-d) unit, and diagenetic unit. Finally, the sedimentological core analysis should be used to calibrate the wire-line logs and associated sursurface data. The main steps in such calibration are to first determine the core-to-log depth correction and then to determine the level at which genetic units and lithofacies can be recognized on the logs. Such calibration leads to better correlation of sedimentological information to nearby non-cored wells and allows for lateral extension of predictive sedimentological models throughout the subsurface study area.

Abstract Continued gas and condensate discoveries in the Late Cretaceous Winters Sandstone of the southern Sacramento Valley provided impetus for detailed paleoenvironmental analyses of that formation. The Winters Sandstone interval, cored in the Cities Service Nixon Community No. 1 well, Solano County, California, was analyzed to ascertain its sedimentary and petrographic characteristics and to determine the depositional environment in which it accumulated. The Winters Sandstone core extends from 8855 to 8882.4 feet in the Nixon Community No. 1 well and includes strata divisible into three lithologic units. The lower unit, Unit 1 (8862.5 to 8882.4 feet), is a massive-appearing porous (25%) and permeable (150-950 md) sandstone with nongradational upper and lower contacts with shale. X-ray radiographs reveal numerous subhorizon-tal laminations throughout much of what otherwise appears to be a structureless sandstone. Two vertical sequences based on differences in grain size measurements can be distinguished, indicating vertical stacking of genetic subunits within the sandbody. These sedimentary features, developed within a well-defined sandstone unit, correspond to typical B-facies sandstones of the Mutti-Ricci Lucchi submarine fan model. As such, Unit 1 is interpreted to be turbidites deposited in a deep-water setting. Overlying the sandstone are shale and silty shales divisible into two units. Small diameter burrows of indeterminate origin are abundant, though discernable on polished core surfaces only where highly concentrated. A modest foraminiferal fauna suggestive of slope or deeper depths was also recovered from the shale interval. The middle unit, Unit 2 (8859-5-8862.5 feet), is a horizontally laminated shale with subhorizontally bedded to contorted silty and sandy zones and with small obliquely to vertically oriented sandstone dikes. These sedimentary features suggest deposition from the fine-grained distal end of one or more turbi-dite flows and are typical of the D-facies of the Mutti-Ricci Lucchi submarine fan model. Unit 3, silty “shale” (8855-8859-5 feet) is generally lacking distinct stratification features, being almost massive in appearance. It’s mode of occurrence presumably was from suspension which, together with its included moderately deep water microfauna, suggests an environment devoid of bottom turbidites, probably at slope depths. Unit 3 is classified as a G-facies deposit of the Mutti-Ricci Lucchi model.

Abstract Thick-bedded, massive Winters sandstones are the principal reservoir facies of the Union Island gas field in the southern Sacramento Valley. The field is a structural-stratigraphic trap set up by structural closure against the Stockton Arch fault and by facies changes within the Winters Sandstone. Since the discovery of the field by Union Oil in early 1972, total gas production has been approximately 90,000,000 MCF from 16 wells. Three major facies are recognized in the nearly 190 feet of conventional cores taken from four development wells in the field: (1) thick-bedded, porous (20 to 30%) massive sandstone, (2) inter-bedded thin sandstone and shale, and (3) laminated shale. Features in the cores as well as seismic, paleontologic, and regional subsurface data indicate a deep-water (bathyal) origin for the Winters Sandstone. X-radiographs of apparently “structureless”, massive sandstones at Union Island field exhibit horizontal or cross stratification in approximately 50% of the intervals examined. The stratification and other structures suggest that the massive sandstones as well as the thinner bedded sandstones were deposited by turbidity currents. The Winters sandstones at Union Island field commonly show upward-fining and upward-thinning sequences, are elongate perpendicular to the depositional slope, and thicken toward the basin. They are interpreted to be coalesced channel deposits which formed at the base of the slope on the upper part of a sandy suprafan. Thinly interbedded sandstone and shale that separates the massive sandstones represents channel-abandonment and interchannel-overbank deposits.

Abstract The upper Miocene Stevens Sandstone is a prolific oil producer in the San Joaquin Basin of California. Stevens production is mainly from deep water sandstones which were most commonly deposited by turbidite flows. Although the Stevens has produced for forty years, a resurgence of activity by Tenneco, Gulf, Texaco, and Arco, as well as many independents, has greatly increased the reserves in the Stevens in recent years. Production from the Stevens interval is primarily from turbidite sandstones deposited as part of submarine fan complexes in fan channels and fan lobes and from sands deposited in topographically lower areas on the sea floor. Fractured siliceous shales of the Stevens interval also contribute to production. These shales, also of deep water origin, are laterally-equivalent or slightly younger than the Stevens sandstones. These shales were deposited on the fringes of the fan, on the basin plain, or as drapes on bathymetrically-high areas of the sea floor. Along the eastern margin of the basin where deposition occurred on a relatively-undeformed homoclinal surface, patterns of turbidite sedimentation and facies associations generally conform to the Mutti and Ricci Lucchi or other submarine fan models. However, in the central and western portions of the basin, fan models seem to be inappropriate. Observed relationships between Facies Associations, sandbody geometries and submarine fan subenvironments often appear anomalous when facies interpreted from cores are compared with relationships described by some currently popular fan models. Such anomalous relationships were observed in cores from several fields producing from the Stevens Sandstone. To explain these inconsistencies, an “on-lap” model and a “confinement” model are proposed for some of the observed depositional patterns of the Upper Miocene Stevens Sandstones in the San Joaquin Basin. Cores from Paloma, North Coles Levee, Rio Viejo and Tule Elk Fields demonstrate the generally thin bedded nature of Stevens tur-bidites deposited in the western portion of the basin. Fining and thinning upward cycles, as well as coarsening and thickening upward cycles, are observed in the cores. Upward variation in the frequency of interbedded shales within the overall sandstone cycles is demonstrated to be the major cuase of apparent “fining” or “coarsening” upward as observed on logs. Complete and incomplete Bouma sequences and relatively thin massive-appearing to graded sandstones are observed in the cores. Amalgamation of sandstones is common. At Tule Elk Field a significant thickness of trough-cross-bedded sandstones show the effect of deep water traction type currents, a phenomena that has rarely been documented. Superimposed on the facies analyses are the effects of basin bottom topography. An “on-lap” model is defined to describe turbi-dite deposits which lap onto and stack vertically against contemporaneously rising anticlinal structures. Internally these sand-bodies exhibit distinct sedimentation cycles and facies associations characteristic of fan progradation. Externally these sandbodies pinch out crestward, may or may not be lobate- or fan-shaped, and tend to be abnormally thick. The Paloma Field is an example of sediments that fit the “on-lap” model. A “confinement” model is defined to describe deposits of turbidity flows which are confined to bathymetric lows between adjacent (en echelon) anticlines. These deposits, which accumulated in synclinal lows, tend to have an external channel-like morphology but do not necessarily exhibit facies associations commonly ascribed to channels in fan models. Deep-water sediments from Yowlumne Field, Tule Elk Field, and some of the production Elk Hills Field are best explained by the “confinement” model.

Abstract Production of gas and some condensate from fine-grained fractured sandstone of the Upper Cretaceous Woodbine-Eagle Ford interval at depths of 10,800 to 11,350 ft in central northern Tyler County has provided the impetus for a detailed paleoenviron-mental analysis of the geology in that area. The productive area (Sugar Creek field) is located a short distance south of the Sabine uplift, which was an active positive area previous to, during and following Woodbine-Eagle Ford deposition, and is slightly down-dip from the Lower Cretaceous continental shelf edge as delineated by the Angelina-Caldwell flexure and the Edwards reef trend. The Woodbine-Eagle Ford interval (between the Buda Limestone below and Austin Chalk above) is 150-200 ft thick in the Sugar Creek field area but thins to less than 50 ft thick above the Edwards reef buildup and northward toward the Sabine uplift where it is missing. Southward (down-dip) the interval thickens to greater than 1500 ft within a distance of 15 miles. The Woodbine-Eagle Ford interval in this down-dip position is a mud-dominated clastic wedge. Cores from seven wells in the Sugar Creek field and two down-dip wells were examined in detail. Dark gray, organic-rich, silty shale with thin laminated to ripple-bedded siltstone beds and small siderite nodules comprise most (40% to greater than 80%) of the Woodbine-Eagle Ford interval and contain a microfauna (foraminifera) indicative of outer shelf to upper slope water depth. The reservoir sandstones occur as complex, single to multi-story bodies 15-40 ft thick and are composed of fine- to very fine-grained quartz arenites. As viewed in polished core slabs, the sandstones are mostly “massive-appearing” (without discernible sedimentary structures). Beds are characterized by very sharp (non-gradational) basal contacts (sandstone/shale) with abundant drag marks, flute casts and other sole markings, and by abrupt upper contacts with shale. X-ray radiography of core slabs has revealed a multitude of sedimentary structures in the otherwise “massive” sandstones. Massive to laminated and cross-stratified sandstone is dominate, but ripple-stratification, soft-sediment-deformation and scour features are also present. Burrows and bioturbation are common but confined only to the upper parts of sandstone beds which may be separated by thin (1-2 inch) shale beds. These sedimentary features and their positions within well-defined sandstone genetic units indicate rapid deposition of sand by low- to high-concentration submarine density (turbidity) currents and associated tractive currents. Mud deposition and burrowing of the upper parts of sand beds occurred during quiet periods between the sand pulses. Highly deformed siltstone intervals often are present below the sandstone bodies and indicate rapid loading by sand deposition and/or slumping on unstable slopes. A conglomerate submarine debris flow deposit is also well displayed in one core. Subsurface correlation and mapping of the discontinuous, lenticular sandstone bodies indicate that they are best delineated as a series of coalescing, dip-oriented lobes. Deposition appears mostly to have been as prograding submarine fan lobes, with sediment being channeled from up-dip delta and nearshore deposits across a narrow shelf and through shelf-edge breaks and then dumped downwlope. These basin-filling deposits pro-graded seaward until the sediment source was cut off and subsequent deposition of the Austin Chalk occurred. Although a major erosional unconformity exists above the Woodbine to the north, no such unconformity can be documented above the down-dip Woodbine-Eagle Ford interval in Tyler County.

Abstract The Spraberry Formation forms the heart of a multibillion-barrel oil field in the Midland Basin, west Texas. Production is from very fine sandstones and siltstones that form low-permeability and low-pressure reservoirs across eight counties in west Texas. The Sun Oil Co., Lottie Jalonick #1 core is continuous through the upper 222 feet (68 m) of the Spraberry Formation and is ideal for facies analysis. Four lithofacies comprise the terrigenous clastics of the Spraberry and Dean Formations: (1) cross-laminated, massive, and parallel-laminated sandstone, (2) laminated siltstone, (3) bioturbated siltstone, and (4) black, organic-rich shale. Silty dolomite mudstone is a very minor carbonate component. Though not common in other cores, numerous fining-upward sequences of sandstone overlain by shale and laminated siltstone are common in the Jalonick core. These are interpreted to be deposits from saline density currents, generated in shelf lagoons and salt pans, as they were dispersed beyond channel mouths in the distal fringe of an upper Spraberry submarine fan. Laminated siltstones were deposited out of suspension from saline density interflows.

Abstract Lead-zinc sulfides were deposited during the Devonian in the locality of the Jason prospect, Yukon Territory. Massive and laminated sulfides are interbedded with clastics of the Canol Formation; galena and sphalerite, among other minerals, precipitated on the sea floor from exhalative fluids along the fault margins of a small graben. The depositional graben center consists of a thick sequence of conglomerates, sandstones, and finer clastics. These were derived from a western source, and were deposited largely by turbidity currents in channel and channel-flanking areas. Conglomerates are massive or graded, and some show faint parallel stratification. Sandstones show Bouma-interval sequences and some grooves and flutes. In contrast, the ore area has a thinner sequence which consists dominantly of sedimentary breccias, pebbly mudstones, and shales. Breccias and pebbly mudstones were deposited from debris flows and show a range of plastic mass-flow structures. Stratigraphic thinning and presence of laterally discontinuous and locally derived mass failure deposits indicate the existence of a penecontemporaneous fault zone. A few conglomerates and sandstones are interbedded with the ore horizon, and these and siltstones and shales were likely deposited from large turbidity currents. Involvement of sediments which had been deposited near the bottom of the graben in later mass failures indicates episodic fault movement during sedimentation. The fault zone appears to have positioned a sub-sea surface, geothermal system which formed metal-depositing fluids. Diamond drilling is underway to determine ore reserves. Core 78-30 shows the range of sedimentary structures and bedding styles of the ore zone.

Abstract This core workshop on deep-water clastic sediments was organized to provide participants with an opportunity to view cores from a variety of deep water depositional settings and to demonstrate the application of process sedimentology in the interpretation of depositional environments from the study of cores and associated subsurface data. The studies assembled for presentation in the workshop have dealt with sedimentary sequences which have been interpreted as having formed by deposition of non-calcareous, clastic sediment in relatively deep water and also have been concerned principally with coarser deep-water sediments of such stratigraphic sequences because of their potential as hydrocarbon reservoirs. The notes were organized to provide written discussions of the studies in which the cores were used. In addition it was a principal objective of the organizers that each contribution contain subsurface wireline logs and extensive photographic coverage of the whole-diameter core sequences.