High Porosity Zones Research Articles

Birba Field PVT Variations Along the Hydrocarbon Column and Confirmatory Field Tests

This paper describes a practical application of thermodynamic modeling that indicated that the PVT properties of the reservoir fluid in the Birba field were strongly depth-dependent, explaining the presence of significantly undersaturated oil only 200 m (655 ft) below a gas cap. Field tests confirmed the thermodynamic model to be correct. Furthermore, this paper discusses the options for field development, including the consideration of possible benefits of (developed) miscibility when gas is injected.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal décollement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating. The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30–40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.

Stationary exchange between an infiltrated granular bed and a body immersed in it

Stationary heat and mass outflow from a body in an infiltrated granular bed is studied taking into account the effect of the high-porosity zone near the surface of the body.

Depositional Environments of Silurian Whirlpool Sandstone (Medina) in Western New York: ABSTRACT

The Whirlpool Sandstone (Medina Group), quartzose, white, clean, well-rounded, and well-sorted, is the basal Silurian unit in western New York. Its source was the Queenston delta some distance to the southeast. Sedimentary structures indicate that deposition occurred in a beach-shore face environment. Fragmented phosphatic marine fossils are found throughout the sandstone. Erosion of Upper Ordovician sediments in western New York was minimal and involved only the uppermost green (reduced) portion of the Queenston Shale, which supplied lags of flat, green shale pebbles to the Whirlpool Sandstone. Soft sediment deformation and burrowing in the Queenston substrate (the transitional contact between the red Queenston Shale and red Grimsby Sandstone [Silurian] in Rochester, New York, immediately to the east of the area of Whirlpool deposition) suggest a short period of nondeposition or possibly mild erosion following the deposition of the Queenston Shale. Thus, the Ordovician-Silurian boundary in western New York is essentially conformable and signified only by an abrupt influx of clean quartz sand. Petrographic analysis shows that the Whirlpool is a second or multicycle sandstone composed primarily of quartz grains originally derived from igneous and metamorphic terranes with only a minor sedimentary component. Because the sandstone is cemented by quartz overgrowths, usually in an advanced stage of development, primary porosity is reduced and the overall porosity of the Whirlpool Sandstone is low (less than 5%). Higher porosities are attributed to the development of secondary moldic porosity caused by dissolution of detrital quartz grains and their original calcite cement. These zones of higher porosity are the most prolific natural gas producing areas in the Whirlpool Sandstone when hydraulic fracturing is used. End_of_Article - Last_Page 1174------------

Detailed Reservoir Geology--Basis for Enhanced Recovery Model, Wasson San Andres Field, West Texas: ABSTRACT

Core studies in the CO2 pilot area of Shell's Denver unit in the Wasson San Andres field of west Texas revealed two basin end-member rock types: pelletal packstones, which exhibit high porosity and permeability due to the effective inter-pellet pore fabric; and moldic wackestones, which have lower porosity and significantly lower permeability, due to the disconnected fabric of the moldic pores. The majority of the rock section consists of a mixture of these end-member rock types due to cyclical variations in the degree of organic burrowing, which created the pellet packstones by reworking the original wackestone lithology. The high quality pellet packstones are dominant in rocks with greater than 15% porosity. The geologic model of the pilot area thus consists f numerous correlative high porosity zones composed dominantly of packstones, interbedded with poorer quality moldic wackestones. An examination of cores from representative wells throughout the Denver unit also documented the occurrence of pelletal packstones, dominantly in rocks with greater than 15% porosity. By using digitized sonic logs from 688 wells in the Denver unit and the LOGPAK program, isopach mapping of packstone thickness in each of the correlative field-wide San Andres subzones was accomplished. Recognition of the detailed zonation of rock types from the pilot area in wells throughout the major part of the Denver unit permitted expansion of the pilot area model to the larger Denver unit model. End_of_Article - Last_Page 932------------

Wall Effects in Permeameters

The effect of a permeameter wall on the mean flow velocity measured in a permeameter containing granular material is investigated. The difference between the measured mean flow velocity and a true one-dimensional velocity in an infinite medium is attributed to a much higher than average flow rate in a narrow zone of high porosity immediately adjacent to the wall. An equation for the ratio of measured to true velocity is derived. Data from an experimental investigation into the velocity-hydraulic gradient relationship for coarse granular media at various porosities are combined with the results of wall-zone porosity measurements to predict values of the ratio. Measured values of this ratio for 16-mm diameter glass marbles and 3 / 4-in. river gravel in a 22.5-in diameter permeameter are compared with calculated values.