Presence Of Coal Seams Research Articles

The Carnian (Late Triassic) flora from Lunz in Lower Austria: Paleoecological considerations

A recently completed systematic macromorphological and cuticular analysis of compressed gymnosperm foliage from the famous Carnian (Late Triassic) flora from Lunz in Lower Austria has provided detailed circumscriptions of the individual taxa and new information as to their ordinal and generic classification. Although the fossils represent several lineages of gymnosperms, including Cycadales, Bennettitales, Ginkgoales, it appeared that certain idiocuticular and epidermal features such as sunken stomata and papillate surfaces occur widespread among the taxa. The presence of coal seams indicates that the environmental conditions in the Lunz paleoecosystem were stable for longer periods and allowed for the accumulation of larger amounts of plant material. The formation of peat generally requires special conditions, including a stable, high groundwater table, reduced oxygen supply and low pH values, which are typically found in swamps or peat bogs. Many of the idiocuticular and epidermal features recorded for the Lunz plants can be interpreted as adaptations to ecological conditions characteristic of coal-producing peat swamps, e.g., physiological drought, and thus corroborate the interpretation of the Lunz paleoecosystem as a peat-forming environment.

Fluid and Lithology Prediction within a Coal Sequence using Seismic Attribute Modelling and Analysis (Gippsland Basin)

In the Latrobe Group of the Gippsland Basin (Australia) the seismic response of the reservoir is masked by the presence of coal seams. These coal seams possess a large contrast in their acoustic properties (density and P-wave velocity) to those of the surrounding rocks (sands and shales). This causes strong sidelobe interference and coherent noise that overprints the more subtle sand/shale and porefill responses that we aim to detect. This has prevented reliable delineation of existing fields and the possible discovery of new fields in the area. To study the effect of coal seams on seismic attributes we modelled the seismic response of sand-shale-coal sequences. Synthetic seismograms of a ?coaly sequence? were built from simple models consisting of three to four layers and also from blocked well log data. The synthetic seismograms were computed for these models using convolutional modelling as well as the reflectivity method, which takes into account multiple reflections and mode conversions. Input models created by randomly shuffling the blocked logs were used to analyse the sensitivity and robustness of AVO attributes to variations in rock and fluid properties. Fluid effects were detectable on the far-offset amplitudes for coal sequences with no more than fifteen to twenty percent coal content. For larger amounts of coal, fluid detection becomes ambiguous regardless of the distribution of coal layers in the sequence. A useful attribute for predicting coal content is the near-offset average absolute amplitude. A better attribute for detecting fluid effects can be formed as a linear combination of the far-offset event amplitude and the near-offset average amplitude. These attributes were applied to a 3D prestack depth migrated dataset to characterise the ?coaly sequence? and calibrate it to nearby wells. The combined fluid attribute predicted the porefill in all five wells and indicated possible hydrocarbon extents in the known fields.

Controlling Cement Tops Through Use of Fibre-Based Slurries Reduces Drilling Costs

Abstract When drilling in southern Alberta near the town of Strathmore, operators have historically encountered unpredictable lost circulation during cementing operations on many wells. With the surface casing set above ground water, operators are required to bring cement to surface on all production casings. Costly remedial cement treatments are often required. Openhole logs in the area have shown the presence of numerous coal seams at depths ranging from 20 to 250 m (66 to 820 ft.) below surface. Postcementing temperature logs indicate that the cement top may be found at any one of these coal seams. The presence of the coal seams alone does not, however, appear to definitively indicate the potential for a problem, as evidenced by wells separated by less than a mile that may react completely differently. Currently, there is no way to determine which wells will experience problems so the normal approach has been to bring cement to surface before dropping the top wiper plug and displacing the casing. Unfortunately, this methodology has not proven to be 100% successful, and wells with no losses require the disposal of a significant amount of cement at a substantial cost. Recently, the introduction of specifically sized silicic fibres to the cement blend in conjunction with a revamped cementing procedure has proven to be a cost-effective solution to the problem of low cement tops. This paper will take a look at previous efforts at solving the problem along with alternatives that have been considered and rejected. It also includes data showing the efficacy and economics of the new approach. Introduction Shallow gas drilling in southeastern Alberta poses a unique set of challenges. Because the production rate on shallow gas wells is relatively low and the number of wells required to fully exploit the reserves is quite high, the driving forces behind drilling and completing the wells all revolve around reducing costs and improving efficiency. To put this into perspective, consider the following: the time from spud to rig release is normally less than 24 hours per well. Typical production from a shallow gas well in southeastern Alberta ranges from 50 to 150 mcf/day. In 2001, there were more than 5,000 shallow gas wells drilled in the Western Canadian Sedimentary Basin. One of the challenges that is often encountered in shallow gas drilling is lost circulation while cementing. Lost circulation while cementing creates additional costs in shallow gas drilling in several ways. Low cement tops require top-out cementing and, in some cases, very costly squeeze cementing operations. Cementing programs are often designed with large excess cement volumes in an attempt to achieve cement back to the surface despite losses. When excess cement is returned to surface, it must be disposed of at a substantial cost. Therefore, the ability to predictably eliminate lost circulation while cementing will do away with the cost of top-ups and squeezes, reduce the amount of cement required, and minimize surface disposal costs for cement returned to surface.

Waulsortian-Type Buildups in the Lower Carboniferous of the Bechar Basin, Northwestern Sahara of Algeria, North Africa: ABSTRACT

The carboniferous strata in the Bechar basin can be subdivided into three major groups. The lower group is composed of bioclastic and perireefal carbonates. The middle group is exemplified by carbonate platform deposits that were eroded during episodes of emergence by channels of continental derived sediments. The upper group is represented by terrigenous deposits composed mainly of deltaic, fluvial, and lacustrine deposits, with the sporadic presence of coal seams. The carboniferous Waulsortian-type buildups within the Bechar basin occur in the lower group. They are exposed above the desert floor along a south to north-northwest-trending axis, with the younger buildups located to the south and the older ones to the north. These bioherms are apparently younger than those recognized in Europe and North America. These Algerian bioherms were initiated and persisted during the time interval represented by the conodont gnathodus bilineatus zone (lower Visean-upper Visean boundary). Although they are younger than their European and North American counterparts that are of Tournaisian-lower Visean age, these mounds appear to have formed in similar environmental and tectono-sedimentary conditions. Their areal extent, geometry, and facies relationships suggest that they were limited to a shelf edge. Their deposition environments range from shallow marine to deep sea.