Stratigraphic

Stratigraphic traps where impermeable strata seals the reservoir... 

SWS are useful to obtain direct indications of hydrocarbons (under UV light) and to differentiate between oil and gas. The technique is applied extensively to sample microfossils and pollen for stratigraphic analysis (age dating, correlation, depositional environment). Qualitative inspection of porosity is possible, but very often the sampling process results in a severe crushing of the sample thus obscuring the true porosity and permeability. 

Tar sand deposits are widely distributed throughout the world (Fig. 2) (5,6) and the various deposits have been described as belonging to two types stratigraphic traps and stmctural traps (Table 2 Fig. 3) (7). However, there are the inevitable gradations and combinations of these two types of deposits, and thus a broad pattern of deposit entrapment is beheved to exist. In general terms, the entrapment character of the very large tar sand deposits involves a combination of both stratigraphic and stmctural traps. 

Fig. 3. Types of traps for tar sand deposits, where 0 represents a stratigraphic trap, x, an intermediate between stratigraphic and stmctural/stratigraphic traps H, a stmctural/stratigraphic trap , an intermediate between stmctural/stratigraphic and stmctural traps and A, a stmctural trap.

Figure 2b Subsurface stratigraphic relationship (Universal Oil Prod., 1972).

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-... 

Stratigraphic traps are formed by depositional and sedimentary factors. In such traps the depositional process and the follow-on cementing process, which changes the sediment bed into a rock, create porosity and permeability alterations in geometric forms that provide traps. 

Combination tra/ s—sedimentary trap features that result from both stratigraphic and structural mechanisms. There can be many combinations for stratigraphic and structural traps. An example of such a trap would be a reef feature overlaying a porous and permeable sandstone, but in which the sequence has been faulted (see Figure 2-54). Without the fault, which has provided an impregnable barrier, the hydrocarbons would have migrated further up dip within the sandstone. 

Evaluated for formations or lithological-stratigraphic complexes on the basis of general geological premises and analogies... 

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 new-field wildcat is a well located on a structural feature or other type of trap which has not previously produced oil or gas. In regions where local geological conditions have little or no control over accumulations, these wells are generally at least two miles from the nearest productive area. Distance, however, is not the determining factor. Of greater importance is the degree of risk assumed by the operator, and his intention to test a structure or stratigraphic condition not previously proved productive. 

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. 

Classification by objective Exploration Exploration OOOO well Appraisal well OO Delineation well Q New-structure test Q New-pool test Q Deeper-pool test Q Shallower-pool test Q New-licence block QQ appraisal test Fault block O extension test Appraisal/outpost 0 test (delineation) Stratigraphic test O0 New-field wildcat 0 New-pool wildcat 0 Deeper-pool test 0 Shallower-pool test 0 Outpost or 00 extension test Key well 00 - Group one - Group two Parametric well O0 Core well 00 Prospecting well 00 Exploratory well 00 Exploration O000 well Appraisal well 0O... 

Chi, W. R., Namson, J., and Suppe, J. (1981). Stratigraphic record of plate interactions in the coastal range of eastern Taiwan. Geol. Soc. China Mem., 491-530. 

Pitman, W. C. Ill (1978). Relationship between eustacy and stratigraphic sequences of passive margins. Geol. Soc. Am. Bull. 89,1389-1403. 

Simplified stratigraphic column in the Hokuroku district and correlations with the Oga stratigraphy (Tanimura et al 1983)... 

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