Regional Subsurface Mapping Project

Exploration activity in the petroleum industry is now mainly of this type. Companies may initially send surface geological mapping parties into the field, but ultimately the most thoroughfield studies in a given field area are likely to be done by government survey organizations. Much industry activity is now located in offshore regions for which surface

geological information is, in any case, sparse, unobtainable, or irrelevant. As noted previously, the beds exposed at the edge of a basin may bear little relation to those buried near its center (Fig.1.1).

Regional subsurface work is based initially on geophysi- cal data and subsequently on test drilling. Gravity and aero- magnetic information may provide much useful information on broad structural features, particularly deep crustal struc- tures. Refraction seismic lines may be shot for the same purpose. More detailed structural and stratigraphic data are obtained from reflection-seismic surveys (Fig.1.3; Chap.6), and these provide the basis for all exploration drilling in the early phases of basin development. Deep reflection surveys have profoundly changed our ideas about deformed belts during the last decade. Seismic shooting and processing is now a highly sophisticated process, and its practitioners like Fig. 1.1 Contrasts between

stratigraphic thicknesses and facies at basin center and basin margin. The geology of the deep, hydrocarbon-producing regions of a basin might be quite different from that at the margins, so that surface geology gives little useful information on what lies below. The North Sea Basin is an excellent example. This example is of the Messinian evaporite basin, Sicily (Schreiber et al.

1976)

Fig. 1.2 A typical diamond drill hole (DDH) network across a mining property, a gold prospect in Precambrian metasediments, northern Ontario

to talk about seismic stratigraphy as if it can provide virtually all the answers, not only about structure, but about the stratigraphic subdivision of a basin, regional correlation, and even lithofacies. This is particularly the case where, as is now common, selected areas of a basin are explored using three-dimensional seismic methods. However, the seismic method is only one exploration tool, and its results must be tested against those derived in other ways. For example, test drilling may show that stratigraphic correlations predicted from seismic interpretation are incorrect (Chap.6). Problems of stratigraphic velocity resolution, the presence of low-angle unconformities, and the obscuring effects of local structure can all introduce errors into seismic interpretation. Resolu- tion of bedding units is still relatively crude, as illustrated in the comparison between typical seismic wave forms and the scale of actual bedding units shown in Fig.7.1. Dating and correlation of seismic sequences depend heavily on bios- tratigraphic and geophysical log information from explo- ration wells that are used to constrain and calibrate stratigraphic reconstructions made using sequence principles. Exploration wells, especially thefirst ones to be drilled in a frontier basin, are as valuable in their own way as space probes sent out to study the planets, and it is unfortunate that in many countries the resulting data are not treated with the same respect afforded space information, but are regarded as the private property of the organization that paid for the drilling. In a competitive world, obviously, a company has the right to benefit from its own expenditures but, in the long-term, knowledge of deep basin structure and stratigra- phy belongs to the people and should eventually be made

available to them. In Canadian frontier areas, well samples and logs must be deposited with the federal government and released for public inspection two years after well comple- tion. Two years competitive advantage is quite long enough for any company in the fast-moving world of the oil industry, and after this time period the well records become part of a national data repository that anybody can use—with obvious national benefits. Seismic records are released after five years.

The nature of the stratigraphic information derived from a well is both better and worse than that derived from surface outcrops. It is better in the sense that there are no covered intervals in a well section, and such sections are generally much longer than anything that can be measured at the surface (perhaps exceeding 6000 m), so that the stratigraphic record is much more complete. The disadvantage of the well section is the very scrappy nature of the actual rock record available for inspection. Three types of sample are normally available:

1. Cuttings. These are produced by the grinding action of the rotary drill bit. These generally are less than 1 cm in length (Fig. 1.4) and can therefore only provide infor- mation on lithology, texture, and microfossil content. The North American practice is to collect from the mud stream and bag for examination samples every 10 ft (3 m) of drilling depth. When drilling in soft lithologies, rock may cave into the mud stream from the side of the hole many meters above the drill bit, so that the samples become contaminated. Also, cuttings of different density Fig. 1.3 Example of an interpreted seismic line across the continental

margin of North Africa (Mitchum et al. 1977a). This is a famous section, having been reproduced in many textbooks as one of thefirst examples to be published showing the wealth of stratigraphic detail that could be extracted from the data. Note the presence of several angular

unconformities, buried paleotopography, the internal architectural details of individual stratigraphic units, and the system of standard coded stratigraphic units employed by the authors. AAPG © 1977, reprinted by permission of the AAPG whose permission is required for further use

may rise in the mud stream at different rates, which is another cause of mixing. It is thus necessary to observe thefirst appearance rule, which states that only the first (highest) appearance of a lithology or fossil type can be plotted with some confidence. Even then, depth distor- tions can be severe.

2. Full-hole core (Fig. 1.5). The rotary drill bit may be replaced by a coring tool when the well is drilling through an interval of interest, such as a potential or actual reservoir bed. However, this type of core is expensive to obtain, and it is rare tofind that more than a few tens of meters, perhaps only a few meters, of core are

available for any given hole.

The advantage of a core is that because of the large diameter (usually on the order of 10 cm) it permits a detailed examination of small- to medium-sized sedi- mentary structures. Macrofossils may be present, and trace fossils are usually particularly well seen. The amount of sedimentological detail that can be obtained from a core is thus several orders of magnitude greater than is provided by chip samples. However, it is frus- trating not to be able to assess the scale significance of a feature seen in a core. An erosion surface, for example, may be the product of a local scour, or it may be a major regional disconformity, but both could look the same in a core. Hydrodynamic sedimentary structures might be present, but it may not be possible to interpret their geometry and, except in rare instances where a core has been oriented in the hole, they provide no paleocurrent information. Orientation can sometimes be deduced if the core shows a structural dip that can be determined from regional structural data or dipmeter (formation microscanner) logs.

3. Side-wall cores. These are small plugs extracted by a special tool from the wall of a hole after drilling has been completed. These cores are rarely available to the geol- ogist because they are used in porosity-permeability tests and are disaggregated for caving-free analysis by bios- tratigraphers. In any case, their small size limits the amount of sedimentological information that they can yield.

In addition to the samples and core, each exploration hole nowadays is subjected to an extensive series of petrophysical logging methods, which provide records by direct analog tracing or digitization. The description and interpretation of such logs has been the subject of several textbooks and cannot be treated exhaustively here. Log information is discussed further in Chap. 2. The following are a few pre- liminary remarks discussing the utility of logs in a subsur- face data-collection scheme.

Most geophysical tools measure a single physical prop- erty of the rock, such as its electrical resistivity, sonic velocity, gamma radioactivity, etc. These properties reflect lithology, and can therefore be used, singly or in combina- tion, to interpret lithology. Because measurements with modern tools have depth resolution of a few centimeters they are of great potential value in deriving accurate, depth-controlled lithologic logs free of the problems of sample caving. The response of a single tool is not unique to each rock type; for example, many different formations will contain rocks with the same electrical resistivity, and so it is not possible to interpret lithology directly from a single log type. However, such interpretations may be possible from a Fig. 1.4 Typical well cuttings. Photo courtesy of J. Dixon

Fig. 1.5 Examples of typical core from a petroleum exploration hole (left) and a diamond drill hole (right)

combination of two or more logs, and attempts have been made to automate such interpretations based on computer- ized calculation routines from digitized log records. Unfor- tunately, the physical properties of rocks and their formation fluids vary so widely that such automated interpretation procedures can only be successful if they are adapted to the specific conditions of each basin. They thus lose much of their exploration value, but become of considerably greater importance once an initial scatter of exploration wells has become available to calibrate the logs.

A particularly common use to which geophysical logs are put is in stratigraphic correlation (Sect. 6.2). Log records through a given unit may have a distinctive shape, which a skilled geologist can recognize in adjacent holes. Correlation may therefore be possible even if details of lithology are unknown and, indeed, the establishment of correlations in this way is standard practice in subsurface work. Similarly, log shape is a useful tool in environmental interpretation, though it must be used with considerable caution.

Logs are also of considerable importance in calibrating seismic records—seismic velocities are routinely derived from sonic logs and used to improve seismic correlations (e.g., see Fig.6.20and discussion in text).

Regional subsurface work may lead to the development of a list of petroleum plays for an area. These are conceptual models to explain the local petroleum system, including the history of generation, migration and trapping for petroleum pools. For example, a Devonian reef play, which might be based on the occurrence in Devonian strata of porous, dolomitized reef masses enclosed in mudrock, would pro- vide an ideal series of stratigraphic traps. The development of a petroleum play leads to the evolution of an exploration methodology, which summarizes exploration experience in choosing the best combination of exploratory techniques and the type of data required to locate individual pools within the play area. At this stage, exploration moves from the play stage to the prospect stage. Thefirst wells drilled in an area might have been drilled on specific prospects, with a specific play in mind, but it is rare for early wells to be successful. Prospect development is the next phase in an exploration program.

It is now common practice in petroleum exploration programs to carry out studies of burial history and thermal maturation of the stratigraphic section in a basin. This is based on what are now called backstripping procedures. Subsidence history is reconstructed by computer programs that reverse the process of sedimentation, stripping off each layer in turn, decompacting the remaining section, and cal- culating the isostatic balance. In this way, the component of subsidence due to tectonic effects can be isolated from that caused by the weight of sediment, and this, in turn, can be related to the rheological behavior of the crust underlying the basin. Burial history can be related to thermal history in

order to document the organic maturation of the sediments and, in this way, the timing of petroleum generation and release can be evaluated.

The recognition and correlation of stratigraphic sequences (Chaps.5and6) now forms the core of regional stratigraphic work. This framework is then interpreted to assess regional paleogeographic histories and cycles of local to regional sea-level changes. The latter may be compared to regional and global standards, although this is of less importance to the exploration geologist than the production of a useable regional stratigraphic framework.