Roger Tyler

Geological and hydrological controls on the producibility of coalbed methane

Geological and hydrological comparison of two United States coalbed methane basins, the prolific San Juan Basin and the marginally producing Sand Wash Basin, indicates that coal distribution and rank, gas content, permeability, ground-water flow, and depositional and structural setting are critical controls on coalbed methane producibility. A complex interplay, and moreover, a synergy amongst these controls determines high productivity. This paper proposes a basin-scale explanation for the prolific and marginal production in the two basins and that can be applied to evaluation of coalbed methane potential in coal basins worldwide. High productivity is governed by (1) thick, laterally continuous coals of high thermal maturity, (2) basinward flow of ground water through coals of high rank and gas content orthogonally toward no-flow boundaries (regional hingelines, fault systems, facies changes, and/or discharge areas), and (3) conventional trapping along those boundaries to provide additional gas beyond that sorbed on the coal surface.

Stratigraphic and structural controls on gold mineralization in the Pilgrim's Rest goldfield, eastern Transvaal, South Africa

Abstract Since 1872, approximately 185 metric tons of gold and a significant amount of silver have been won from numerous stratiform ore bodies within the Pilgrims Rest goldfield. The epigenetic stratiform ore bodies are found primarily in the Malmani Dolomite Subgroup, a laterally extensive Lower Proterozoic carbonate platform succession. A strong relationship exists between the presence of stratiform ore bodies and environment of deposition and structural deformation. Analysis of the host sediments indicates deposition in a retrogradational, followed by an aggradational, sequence that comprises supratidal (sabkha) siliciclastic sediments to midshelf carbonates. The retrogradational and aggradational sequences are characterized by numerous higher frequency transgressive and regressive cycles. Mineralization is, with few exceptions, restricted to shallow-water siliciclastic sediments; deeper water carbonates are barren. More importantly, siliciclastic mudstones and sandstones have acted as zones of detachment for thin-skinned thrust-fault deformation, resulting in the development of conformable passageways in which fluids concentrated their auriferous lodes. Evidence of shallow hinterland-dipping duplex, antiformal stack, and imbricate thrust systems abounds in several underground and field exposures. The gold-silver emplacement model for the Pilgrims Rest goldfield infers fluids derived from a deep-seated magmatic source. Oxygen-isotopic compositions of the mineralizing fluids, recalculated from δ18O quartz (+ 12.1‰ to + 19.5‰), point to a magmatic source and subsequent mixing with saline, evolved formation water. A magmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions (100°C/km). The Bushveld Igneous Complex (Rustenburg Layered Suite) to the west of Pilgrims Rest is thought to be the source of the auriferous hydrothermal solutions, and its emplacement was the driving mechanism of thrust-fault deformation in the Pilgrims Rest goldfield.

Differential compaction of interbedded sandstone and coal

Abstract Cretaceous and Tertiary coal beds in the western United States typically contain subvertical opening-mode fractures (cleat). However, closely spaced normal faults abruptly substitute for opening-mode fractures in coal beneath some sandstone lenses having blunt terminations. Differential forced-fold compaction of coal beds around and beneath lens-shaped sandstone bodies accounts for such shifts in fracture style. Finite element modelling of coal deformation shows that shear stress is augmented in coal layers below abruptly tapering edges of sandstones lenses, favouring fault development, whereas under gradually tapering lenses shear stresses are not sufficiently enhanced to cause shifts in fracture style. Upper Cretaceous Mesaverde Group coal beds in southwest Wyoming have significant variations in fracture style over distances of a few to tens of metres. Because these faults have little or no porosity, the coal that contains them is likely to have low permeability compared to coal having typical (generally porous) opening-mode fractures. Thus, shifting fracture style may affect regional and local gas and water flow in coal beds.

Reactivation of mature oil fields through advanced reservoir characterization: A case history of the Budare field, Venezuela

Budare field has produced 95 million bbl of oil since discovery in 1954, but a sustained 6 yr decline during the early 1990s reduced daily production to 3000 bbl of oil. Reactivation of the field as a result of this reservoir characterization study increased production by 13,000-16,000 BOPD, a rate that has been maintained in the 4 yr since the study was completed, resulting in an incremental recovery of more than 24 million bbl of oil. This increase in production was achieved through integrated reservoir characterization that identified the depositional heterogeneities and structural complexities responsible for intrareservoir entrapment of the bypassed oil in the field. The main producing zones are the Tertiary-age Merecure and Oficina reservoirs that are interpreted as the deposits of large-scale bed-load and mixed-load fluvial and wave-dominated deltaic depositional systems. The geologic analysis indicates that the large-scale systems are divided internally, or vertically stratified, by thin but widespread shale markers resulting from flooding episodes and that facies variability introduces lateral discontinuities. Syn- and postdepositional faulting further disrupts reservoir continuity. Trends in fluid flow established from engineering analysis of initial fluid levels, response to recompletion workovers, and pressure depletion data demonstrated that these geologic heterogeneities (flooding shale markers, lateral facies pinch-out, and faults) are effective barriers to lateral and vertical fluid flow. Considerable potential for sustained production exists at Budare field because the reservoir units are highly compartmentalized. Identification and targeting of the poorly drained and uncontacted compartments at Budare facilitated the development of a production optimization portfolio that encompassed four principal advanced-recovery opportunities: field extension or step-out; attic areas of the reservoir that are structurally higher than existing production and, hence, poorly drained; stratigraphically and structurally defined compartments that have not been tapped; and compartments that are poorly drained. Successful geologically targeted infill wells and strategic recompletions in these bypassed compartments achieved a sustained fivefold increase in daily production in the mature Budare field.

Defining Coalbed Methane Exploration Fairways: An Example from the Piceance Basin, Rocky Mountain Foreland, Western United States

A basin-scale coalbed methane producibility and exploration model has been developed on the basis of research performed in the San Juan, Sand Wash, Greater Green Rivers, and Piceance Basins of the Rocky Mountain Foreland and reconnaissance studies of several other producing and prospective coal basins in the United States and worldwide. The producibility model indicates that depositional setting and coal distribution, coal rank, gas content, permeability, hydrodynamics, and tectonic/structural setting are controls critical to coalbed methane production. However, knowledge of a basin’s geologic and hydrologic characteristics will not facilitate conclusions about coalbed methane producibility because it is the interplay among geologic and hydrologic controls on production and their spatial relation that govern producibility. High producibility requires that the geologic and hydrologic controls be synergistically combined. That synergism is absent in the marginally producing, hydrocarbon-overpressured Piceance Basin. As predicted from the coalbed methane producibility model, significant coalbed methane production (greater than 1 MMcf/d [28 Mm3/d]) may be precluded in many parts of the hydrocarbon-overpressured Piceance Basin by the absence of coalbed reservoir continuity, high permeability, and dynamic groundwater flow. The best potential for coalbed methane production may lie in conventional and compartmentalized traps basinward of where outcrop and subsurface coals are in good reservoir and hydraulic communication and/or in areas of vertical flow potential and fracture-enhanced permeability. In the low-permeability, hydrocarbon-overpressured Piceance Basin, exploration and development of migrated conventionally and hydrodynamically trapped gases, in-situ-generated secondary biogenic gases, and solution gases will be required to achieve high coalbed methane production.