Richard K. Vessell

Porosities, Permeabilities, and Microfabrics of Devonian Shales

Shales generally are regarded as source rocks and seals. However, in the Illinois, Michigan, and Appalachian Basins, shales of Devonian age commonly are regarded as reservoir rocks. According to Broadhead et al. (1982), economic gas production from nonfractured shales has been an established fact in these basins for several decades. In some areas, natural fractures are an important control on production. In other areas, gas productive shale intervals lack natural fractures.

Dimensions and quality of reservoirs originating in low and high sinuosity channel systems, Lower Cretaceous Travis Peak Formation, East Texas, USA

Abstract Quantification of the dimensions and quality of fluvial reservoirs requires knowledge of channel style, depositional environment and diagenesis. Two styles of fluvial channel co-existed during the rapid aggradation of >600 m of Lower Cretaceous, Travis Peak sediments in East Texas: (1) high sinuosity (meandering) channels, and (2) low sinuosity (straight) channels. Each was restricted to a specific geographic area, the result of long-term, geomorphic stability. In both channel systems, reservoir sandstones originated principally in channel and crevasse splay environments. Reservoir models developed through sedimentological analysis are similar to those developed independently through reservoir simulation studies. Reservoir sandstones originating as point bars in high sinuosity channel systems are relatively small (1.2 km2 or less), thin (3.6 m), heterogeneous, and isolated within non-pay mudrocks. Reservoir sandstones originating as medial and transverse/oblique bars in the low sinuosity system are areally extensive (>20 km2) thick (3.6 to 13.7 m), homogeneous, and display complex pressure relationships due to avulsion of long stream segments, and the lateral and vertical stacking of successive channels in well-defined channel belts. The greatest volume of channel sandstone occurs in the low sinuosity channel system. The high sinuosity system is dominated by overbank deposits. Travis Peak sandstones have been extensively modified by compaction and cementation. Despite extensive diagenesis, permeability values reflect original depositonal environment and bedding style, even in rocks which have lost more than 80% of their original, depositional porosity. Channel sandstones have higher permeability than associated splay sandstones. Within a channel sandstone, the highest values of permeability occur in destratified, and flat- to low-angle cross-bedded sandstones: planar cross-bedded and ripple-bedded sandstones have the lowest values of permeability. Original depositional environment and bedding style exercise important control on permeability (particularly potential gas flow) even in rocks in which the pore systems have been modified significantly by diagenesis. A knowledge of depositional environment and bedding style is therefore important in predicting potential producibility in tight, gas sandstones such as the Travis Peak formation.

Application of Integrated Reservoir Management and Reservoir Characterization to Optimize Infill Drilling

Initial drilling of wells on a uniform spacing, without regard to reservoir performance and characterization, must become a process of the past. Such efforts do not optimize reservoir development as they fail to account for the complex nature of reservoir heterogeneities present in many low permeability reservoirs, and carbonate reservoirs in particular. These reservoirs are typically characterized by: o Large, discontinuous pay intervals o Vertical and lateral changes in reservoir properties o Low reservoir energy o High residual oil saturation o Low recovery efficiency

Application of Integrated Reservoir Management and Reservoir Characterization to Optimize Infill Drillings. Annual technical progress report, June 13, 1996 to June 12, 1998

Infill drilling of wells on a uniform spacing, without regard to reservoir performance and characterization, does not optimize reservoir development because it fails to account for the complex nature of reservoir heterogeneities present in many low permeability reservoirs, and carbonate reservoirs in particular. New and emerging technologies, such as geostatistical modeling, rigorous decline curve analysis, reservoir rock typing, and special core analysis can be used to develop a 3-D simulation model for prediction of infill locations. Other technologies, such as inter-well injection tracers and magnetic flow conditioners, can also aid in the efficient evaluation and operation of both injection and producing wells. The purpose of this project was to demonstrate useful and cost effective methods of exploitation of the shallow shelf carbonate reservoirs of the Permian Basin located in West Texas.

From the pore scale to reservoir scale: Lithohydraulic flow unit characterization of a shallow shelf carbonate reservoir, North Robertson Unit, West Texas

This paper presents the results of integrated geological-petrophysical reservoir characterization performed as part of the US Department of Energy Class II reservoir program. Petrographic image analysis, using a specially equipped SEM, allowed for the identification of 8 petrophysical rock types at the North Robertson Unit. Detailed log analysis resulted in the development of algorithms for the log-based identification of these rock types in 109 wells. Porosity was related to permeability for each Rock Type: thus permeability is determined from well log data. Evaluation of porosity, permeability, Sw and HPV distribution has allowed for the identification of 12 lithohydraulic flow units. These flow units have been mapped across the unit. The technique allows for the development of log-based reservoir models that are simulator-ready. The results of this study have application to all heterogeneous, shallow shelf carbonate reservoirs, they demonstrate that large fields can be successfully characterized using few cores and emphasize the importance of integrated geological-engineering analysis in reservoir characterization.

Reservoir Models for Meandering and Straight Fluvial Channels; Examples from the Travis Peak Formation, East Texas

ABSTRACT Reservoir geometry and heterogeneity are fundamentally controlled by channel style. Two channel styles existed during Travis Peak deposition: (1) meandering (high sinuosity) channels, and (2) straight (low sinuosity) channels. Each is restricted to a specific geographic area, a result of the long term stability of the loci of sediment input into the ancestral Gulf of Mexico. Reservoir models for meandering and straight fluvial systems have been developed from cores and logs using knowledge of depositional environment and channel style. Predictions of reservoir dimensions in the Travis Peak Formation are based on geologic modeling. The geologic models are supported by reservoir simulation studies performed as part of several staged field experiments sponsored by the Gas Research Institute. Reservoirs that are developed in high sinuosity systems originate as point bars and as crevasse splay deposits. They are isolated within flood basin mudrocks. Point bar reservoir sandstones average 12 ft in thickness and cover approximately 300 acres. Point bar sandstones are usually compartmentalized by mud drapes, particularly in their upper portions. The drapes are sand-rich, and do not appear to act as barriers to fluid flow when wells are fracture stimulated. Crevasse splay sandstones are laterally extensive (>640 acres) but have lower reservoir quality and are thinner than associated point bar sandstones. Reservoirs developed in the low sinuosity system originate as in-channel braid bars and as crevasse splays. Channel sandstones are lithologically homogeneous. The average thickness of a single, low sinuosity channel sandstone is 8 ft. Such deposits are rare because successive channels stack vertically. As a result of channel avulsion and vertical stacking, reservoir sand bodies are very large (at least 5000 acres) and thick (12 to 45 ft). Pressure relationships between successive channels and associated crevasse splay deposits are complex. Wells drilled in Harrison and northeast Panola counties penetrate high sinuosity channel sands while wells drilled in the rest of the study area penetrate low sinuosity channel sands. Within each area, sand bodies originating in channel and crevasse splay environments can be differentiated using a variety of wireline log responses. The volume of sand (channel sand, and channel plus overbank sand) is significantly greater in the low sinuosity system than in the high sinuosity system throughout the whole Travis Peak interval. However, the percentage of sand in the Travis Peak section increases with increasing depth in both areas. In terms of resource potential, the lower portions of the Travis Peak are extremely important because these sands are gas-saturated.

Geologic controls on reservoir properties in gas-bearing middle and Upper Devonian rocks, southern Appalachian basin

Porosities and permeabilities have been measured for a wide range of nonfractured Devonian lithologies in 23 wells from southeastern Ohio, eastern Kentucky, West Virginia, and Virginia. These reservoir properties can be related directly to the geometry of the pore system. Pore geometry, in turn, is a function of rock lithology and mineralogy. Despite the lithologic complexity of the Devonian sequence, reservoir quality can be related to a small number of differing pore geometries.

Sedimentology of Volcaniclastic Deposits from 1971-1974 Eruption Cluster of Volcano Fuego, Guatemala: ABSTRACT

Volcaniclastic sedimentation during and subsequent to the 1971-74 eruption cluster of the Volcano Fuego in Guatemala has occurred in four distinct phases which are part of a 15 to 25 year cycle of sedimentation. In phase 1, the eruption cluster generated 6 × 108 cu m of tephra, one-third in the form of glowing avalanches, the remainder as an elongate airfall ash blanket west-southwest of the cone. Glowing avalanches with a volume of 5 × 107 cu m formed two fans, each 1 to 3 m thick, east and west of the crater. Further avalanches flowed down seven narrow canyons radiating to the south of the crater forming 40-m-thick deposits totaling 1.3 × 108 cu m. During phase 2, debris flows and flash floods removed about one-third of th phase 1 canyon deposits in the first 2 years following eruption. Fan deposits remained intact. Three digitate, 1 to 2.5-m-thick, debris-flow deposits (2.2 × 107 cu m) and two 1-m-thick flood fans (1.8 × 107 cu m) formed south of the crater. In phase 3, terraced, meandering, suspended-load streams were metamorphosed to braided, aggrading, bed-load systems annually eroding 6 million tons of phase 1 and 2 debris, primarily from the canyon deposits. Transport of about two-thirds of this debris to the sea has produced rapid coastal progradation. During phase 4, 15 to 25 years of phase 1 and 2 activity will remove canyon avalanche deposits, redistributing the material in stable fans on the lower volcanic slopes. Phase 1 and 2 processes become inactive while str am incision produces discontinuous terracing. Fluvial systems return to meandering, suspended-load streams. End_of_Article - Last_Page 843------------