Anthony J. Lomando

The Influence of Solid Reservoir Bitumen on Reservoir Quality (1)

Reservoir bitumen precipitates in pore systems from the alteration of trapped or migrating oil in carbonate and siliciclastic reservoirs. Reservoir bitumen is not readily identified on typical log suites, where it is read as open porosity, which has hampered its broad recognition as a reservoir-altering cement. When present as a solid, it can be as influential on reservoir quality as carbonate, silica, or authigenic clay cements and should therefore be evaluated as part of the diagenetic evolution of any pore system whenever encountered. Five morphotypes of reservoir bitumen have been identified and named: droplets, carpets, peanut brittle, vesicular, and digitate. Data from east Texas, Gulf Coast, and west African examples indicate that reservoir bitumen can significantly reduce total effective porosity. Permeability can be significantly reduced by restricted or closed pore throats and by fines migration, even when bitumen occurs in only moderate amounts. The occurrence and distribution of bitumen on well, field, and regional scales can range from uniform and predictable to irregular and unpredictable but mappable through examination of core and cuttings. Within a trap, bitumen can cause heterogeneity and form permeability barriers not related to depositional facies or pre-bitumen diagenesis and reservoir quality. It economic importance must be recognized when reservoir parameters are adversely affected, impacting reserves calculations, recovery factors, and secondary recovery programs. On a regional scale, predictive models can be developed to determine the areas of a trend or portions of a stratigraphic section that are most likely to be affected, and these models can be incorporated with other data for evaluating expected reservoir quality in regional play assessments.

Holocene cemented beach deposits in Belize

Abstract Two types of cemented beach deposits occur on reef islands off the coast of Belize. These are (1) intertidal beachrock that is dominantly cemented by marine aragonite and high-magnesium-calcite cements, and (2) supratidal cayrock that is cemented mainly by vadose low-magnesium-calcite cements. Besides differences in position relative to present sea level and resulting early diagenesic features, beachrock and cayrock can be distinguished on the basis of differences in composition, texture, geographical position, and age. Whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments, cayrock is enriched in benthic foraminifera. Intertidal beachrock is moderately to well sorted and well cemented, while supratidal cayrock is very well sorted, poorly cemented and friable. Beachrock occurs preferentially on windward beaches of sand-shingle Gays on the middle and southern barrier reefs and on the isolated platforms Glovers and Lighthouse Reefs. Cayrock only occurs on larger mangrove-sand Gays of the isolated platforms Turneffe Islands, Lighthouse Reef, and the northern barrier reef. 14C-dating of ten whole-rock and mollusk shell samples produced calibrated dates between AD 345 and AD 1435 for beachrock and between BC 1085 and AD 1190 for cayrock. The large-scale distribution of beachrock in Belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution. Only on windward sides of cays that are close to the reef crest, where large amounts of seawater flush the beaches, considerable amounts of cements can be precipitated to produce beachrock. Cayrock forms due to cementation in the vadose zone and is only preserved on larger, stable mangrove-sand cays.

Last interglacial reef growth beneath Belize barrier and isolated platform reefs

We report the first radiometric dates (thermal-ionization mass spectrometry) from late Pleistocene reef deposits from offshore Belize, the location of the largest modern reef complex in the Atlantic Ocean. The results presented here can be used to explain significant differences in bathymetry, sedimentary facies, and reef development of this major reef area, and the results are significant because they contribute to the knowledge of the regional geology of the eastern Yucatan. The previously held concept of a neotectonically stable eastern Yucatan is challenged. The dates indicate that Pleistocene reefs and shallow-water limestones, which form the basement of modern reefs in the area, accumulated ca. 125–130 ka. Significant differences in elevation of the samples relative to present sea level (>10 m) have several possible causes. Differential subsidence along a series of continental margin fault blocks in combination with variation in karstification are probably the prime causes. Differential subsidence is presumably related to initial extension and later left-lateral movements along the adjacent active boundary between the North American and Caribbean plates. Increasing dissolution toward the south during Pleistocene sea-level lowstands is probably a consequence of higher precipitation rates in mountainous southern Belize.

Isolated carbonate platforms of Belize, Central America: sedimentary facies, late Quaternary history and controlling factors

Abstract The closely spaced, isolated carbonate platforms of Glovers Reef, Lighthouse Reef and Turneffe Islands (Belize) differ significantly with regard to geomorphology and distribution of sedimentary facies, especially in platform interiors. Glovers Reef has a deep (18 m) lagoon with 860 more or less randomly distributed patch reefs. The interior of Lighthouse Reef is characterized by a linear trend of hundreds of coalescing patch reefs that separate a deeper (8 m) eastern and a shallower (3 m) western lagoon. Within both Glovers and Lighthouse Reefs, non-skeletal (peloidal) wackestone and packstone are found in shallow (<5 m) water depths. Deeper lagoon parts are characterized by mollusc-foram wackestones. Facies belts are circular in Glovers Reef, as opposed to linear in Lighthouse Reef. The Turneffe Islands platform has up to 8 m deep interior lagoons that are surrounded by large land/mangrove areas. These lagoons have restricted circulation, are devoid of coral patch reefs, and are dominated by organic-rich wackestone. These differences are largely a consequence of variations in antecedent topography and in exposure to waves and currents from platform to platform. Differences in elevation and relief of the Pleistocene basement are most likely controlled by differential subsidence and latitudinal variation in karstification. Differences in exposure to waves and currents are created as the Turneffe Islands platform is in a leeward position, protected from the open Caribbean Sea by Lighthouse Reef to the east. Variation in Pleistocene elevation caused the platforms to flood successively at different rates during Holocene sea-level rise. However, all reefs investigated kept pace with the rising Holocene sea level. Sediment is now largely bypassing the margins and filling in platform interior lagoons. These examples indicate how local attributes of antecedent topography and exposure to waves and currents can be at least as important as globally operating factors such as sea-level fluctuations. This observation should be kept in mind when interpreting palaeo-relationships in the geologic record.

Giant oil and gas fields : a core workshop

Giant fields tend to be grouped as a distinctive class of hydrocarbon targets, but giant reservoirs are vastly different from one another with varying factors controlling rock type, facies, porosity evolution, and trap mechanism, even in adjoining reservoir zones. As such the ideas generated from the geological analyses of these giants can be applied in the many basins of the world to exploration and production targets of any size. The papers presented in these volumes are examples of giant fields from North America, the North Sea, Middle East, and Indonesia. We have been fortunate enough to put together a group of papers which spanc the major range of geologic time and have organized them accordingly. This sense of diversity extends through the major characteristics represented by these examples of giant reservoirs. A good mix of siliciclastic and carbonate rock types deposited in fluvial and supratidal settings down to deep marine environments can be found among these papers. Trap types range from simple and complex structural to stratigraphic and combination traps. We hope that this group of papers will inform and stimulate the reader to develop new idea and approaches to explore for and develop fields of all types and sizes. Giant fields tend to be grouped as a distinctive class of hydrocarbon targets, but giant reservoirs are vastly different from one another with varying factors controlling rock type, facies, porosity evolution, and trap mechanism, even in adjoining reservoir zones. As such the ideas generated from the geological analyses of these giants can be applied in the many basins of the world to exploration and production targets of any size. The papers presented in these volumes are examples of giant fields from North America, the North Sea, Middle East, and Indonesia. We have been fortunate enough to put together a group of papers which spanc the major range of geologic time and have organized them accordingly. This sense of diversity extends through the major characteristics represented by these examples of giant reservoirs. A good mix of siliciclastic and carbonate rock types deposited in fluvial and supratidal settings down to deep marine environments can be found among these papers. Trap types range from simple and complex structural to stratigraphic and combination traps. We hope that this group of papers will inform and stimulate the reader to develop new idea and approaches to explore for and develop fields of all types and sizes.

Sedimentation and Diagenesis of Upper Smackover Grainstone, Jay Field Area, West Florida: ABSTRACT

The examination of core and logs from a well 3 mi (5 km) southwest of the Jay field has given considerable insight into the upper Smackover facies distribution, diagenesis, and the application of recent models for the sedimentation and diagenesis of this Jurassic reservoir. A 63-ft (19 m) thick unit of oolitic and oolite-oncolite grainstones is recognized in the upper Smackover. High-angle inclined bedding, visible on both core and dipmeter, with a consistent 15 to 20° northeast dip, demonstrates the presence of oolite bars. These bars formed a barrier which affected subsequent deposition and diagenesis in the Jay field area. A complicated diagenetic history of marine and vadose cementation, and pervasive and selective dolomitization have left a unique imprint on the porosity and permeability of these rocks. Dipmeter results and petrographic analysis of the grainstones indicate that cementation and diagenesis have not been uniform. Within the large-scale cross-strata, permeable beds are interstratified with tightly cemented or compacted, impermeable beds. Horizontal flow should be greatest along the strike of the inclined units, because the flow would remain within the permeable planes of the inclined strata. Thus, dipmeter correlation permits an interpretation of the direction of bedding permeability anisotropy produced by the inclination of the pore system. The characteristics of sedimentation and facies distribution in the Jay field area have previously been compared with a modern analog from Joulters Cay in the Bahamas. The Trucial Coast of the Persian Gulf in the Abu Dhabi region may be a better model. The style of deposition and distribution of carbonate and evaporite sediments, and diagenetic characteristics in the grainstone barriers and lagoons closely fit the sedimentation and diagenetic pattern in the Jay Field area. End_of_Article - Last_Page 950------------

Carbonate Structural and Stratigraphic Trap with a Diagenetic Twist: West Purt Field, East Texas: ABSTRACT

The Rodessa Limestone (Aptian) produces from structural and combination traps in the East Texas basin. West Purt field, in northeastern Anderson County, is a Rodessa combination trap where porosity and permeability have been affected by hydrocarbon alteration, adding an additional complexity to the reservoir. In West Purt field, porous skeletal grainstones successively pinch out obliquely across the crest of a northwest-plunging structural nose. The structure is cut by a eastward-dipping fault that forms the eastern boundary of the field. The reservoir grainstones have been subdivided into 3 facies. Two of these facies are fine-grained to cobble-size, poorly sorted coral-skeletal rudstone, cyclicly interbedded with fine-grained, well-sorted mollusk-echinoid grainstone and packstone. The third facies is the overlying fine to coarse-grained mollusk-peloid grainstone, commonly laminated or graded. The overall sequence is interpreted as a prograding shoreface and foreshore deposit. Among the more significant aspects of diagenesis are the early formation of moldic porosity that is partially filled with phreatic isopachous and equant calcite spar cements. Later compaction and minor cementation by saddle dolomite and anhydrite had a minimal effect on porosity. The final stage of cementation was the precipitation of solid bitumen. This bitumen causes a moderate decrease in core-measured porosity, but a significant decrease in permeability by plugging pore throats. The presence and distribution of solid bitumen are not discernible on logs owing to the lack of significant density contrast between crude oil and bitumen. Solid bitumen occurs only in wells adjacent to the eastern boundary fault, regardless of structural elevation. Geochemical analyses of bitumen samples uggest that secondary gas from an underlying source (migrating up the eastern boundary fault) caused the precipitation of solid bitumen by deasphalting the in-place oil. End_of_Article - Last_Page 280------------