Stratigraphic drilling

Seismic studies are very useful when information is lacking on subsurface stratigraphy, or when the depth to bedrock, the water table, a particular bed or formation, or some other layer in the subsurface must be determined without drilling. Seismic lines can be set up to produce stratigraphic cross sections and they can be a very useful "non-invasive" inves-... 

A shallow stratigraphic test is a relatively shallow well drilled to provide information on the local geology, i.e. structure, facies, geochemistry, etc. Normally its immediate purpose is not exploration for oil or gas accumulations. 

A deep stratigraphic test is drilled to obtain information about a specific geological condition that might lead to the discovery of an accumulation of hydrocarbons. Such wells are customarily drilled without the intention of being completed for hydrocarbon production. This classification also includes tests identified as core tests and all types of expendable holes related to hydrocarbon exploration. 

A development well is a well drilled within the proved area of an oil or gas reservoir to the depth of a stratigraphic horizon known to be productive. 

The test drillings will define the stratigraphical units in the area while the geophysics and geological mapping are used for extrapolation of the layers and for definition of geometry, see Figure 43. 

Bezys, R.K. 2000. Stratigraphic investigations and corehole drilling program, 2000. In Report of Activities 2000, Manitoba Industry, Trade and Mines, Manitoba Geological Survey, 196-201. 

Several sections of economic Uranium mineralization (>0.1% UsOs) were identified in drill-core. These appear to be associated to discrete stratigraphic horizons and to fault zones, of which the latter are strongly suspected to have acted as conduits of hydrothermal fluids. 

On ODPLeg 164, three sites were drilled below the base of hydrate stability over a short distance (9.6 km) in the same stratigraphic interval. Figure 7.21b shows the three Leg 164 holes Site 994 without a BSR, Site 995 with a weak BSR, and... 

Answer 7.4 Date of drilling, water horizons encountered as drilling progressed, their temperature and composition, intervals of loss of circulation fluids, depth of the well, the lithological and stratigraphic profile, perforation intervals, initial hydraulic head and temperature of the pumped water, rise of the water table after the water was first encountered by the drillers (indicating confinement), and more. This is indispensable information that justifies every effort needed to get the drillers reports. 

Gibson et al., 1991b). Their stratigraphic position is plotted in the reference section in Figure 2. Recovery from all of these drill cores was found to be -100%. 

On Maliata the stratigraphic thickness of the accreted plateau reaches 3-4 km, and the succession is dominated by pillowed and massive basaltic flows (Petterson et al., 1997 Babbs, 1997). Like the CCOP, dykes are volumetrically minor. The DSDP/ODP drill holes have penetrated into the plateau to a depth of 216 m (Site 1185B Mahoney et at, 2001). The sampled sections consist predominantly of pillowed and massive basalts with occasional thin interlava sediments (Neal et al., 1997 Mahoney et al., 2001)... 

Fig. 10-9. Frequency distribution of helium in groundwater, Yccrlirric area. Western Australia (A) data from stock wells, bores and shallow drill holes (n=70) and (B) data from slotted, cased stratigraphic bore holes (n=50) (from Butt and Gole, 1986).

Several types of information were used to assess stratigraphic constraints on arsenic concentrations in well water. Water level and well constmction data were obtained from well records. Regional correlations of stratigraphic units were inferred from field mapping and drill core. 

Even in Pennsylvania, where Professor H.D. Rogers indicated that several of the successful new wells were located on anticlines less than a year after the drilling of Drake s well, such information was ignored. Of course, it is always easy to ridicule this primitive approach. However, undoubtedly many geological advisors were quite unreliable. Moreover, if the structural style is not too complicated, empirical rules often work. Trends of rivers frequently played a part in predictions and these are often parallel to the structural strike. Another characteristic of early exploration efforts was the technical difficulty of drilling to any considerable depth and the poor recording of stratigraphic detail, which both hamper the subsurface analysis. 

The study area comprises 28,000km in southeastern Alberta (Fig. 1). Rocks of interest to this study range in age from Tertiary to Carboniferous. The stratigraphic sequence, however, does contain older Paleozoic and Precambrian rocks at depth. With the exception of certain carbonate rocks in the Upper Paleozoic, the dominant lithology is shale with occasional sandstone units and minor coal. Some of the sandstone units contain substantial quantities of oil and gas. As a result of continued exploration for and development of these resources, a tremendous quantity of conventional lithologic log, geophysiced log, core and drill-stem test data is available for this area. 

The discovery well intersected an 8 2 m net gas column in Upper Angel Formation sandstones. Three appraisal wells established major hydrocarbon reserves at the same stratigraphical horizon, with a maximum gas column of 140 m (Vincent Tilbury, 1988). A thin oil leg was discovered beneath the gas cap in all four wells, but is thickest at Angel-3 (20 m), the only well drilled south of a major ENE-WSW-oriented strike-slip fault zone that traverses the field (Ryan-Grigor Schulz-Rojahn, 1995). The gas composition in Angel Formation reservoirs consists of dominantly methane (80% by molar volume), ethane (6%), propane (3%) and heavier hydrocarbon gases (up to C7+), including minor carbon dioxide (less than 3%) (Woodside, 1971, 1972, 1990). 

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