In a large unexplored territory, regional geologic synthesis can be a guide in selecting areas on which to expend available money, time, and effort for petroleum exploration. North America has been more thoroughly explored for oil and gas than any other continent; hence, a review of its geology with reference to occurrence of petroleum can be useful in a discussion of general regional evaluation.

Major North American geologic provinces include: (1) a craton or central stable region, (2) orthogeosynclines which surround or nearly surround the craton, (3) local sedimentary basins which have developed upon the orthogeosynclines, and (4) marginal coastal plains.

The craton, or central stable region, contains (a) a vast shield primarily composed of Precambrian igneous and metamorphic rocks and (b) wide margins and interior patches bearing sedimentary rocks dating from Precambrian to Recent, largely undisturbed. The sedimentary rocks of the craton constitute one of the major petroleum provinces of North America.

The orthogeosynclines (Appallachian-Ouachitan-Marathon, Cordilleran, and Franklinian) are geologically complex areas at the borders of the continent composed of miogeosynclinal and eugeosynclinal belts. Miogeosynclines are craton-marginal and in part are petroleum bearing. They are generally flanked by eugeosynclines which are virtually non-petroliferous.

Basins developed upon old eugeosynclines in North America include: (a) Mississippian and Pennsylvanian rocks of the St. Lawrence Gulf region which have yielded minor production; (b) the Tertiary strata of southern California (abundantly productive), similar but currently less productive basins northward along the Pacific Coast; and (c) the Sverdrup basin in the Arctic Archipelago which may contain large reserves of oil and gas. Also superposed on orthogeosynclines are coastal plains of the Atlantic, the Gulf of Mexico, and the Arctic.

Tertiary and Cretaceous strata of the western Gulf Coastal plain comprise the most prolific petroleum province in North America. The Arctic Coastal plain contains oil and gas in Alaska but has not been explored in Canada. The east Gulf and Atlantic plains await further search.

Extra-cratonic Tertiary (and closely associated older) rocks in the Gulf Coast and California have yielded more oil and gas than rocks of all other ages in North America, but the craton has nevertheless produced very large quantities from strata of all ages younger than Precambrian in a wide array of trap types extending over half the continent.

Regional geologic synthesis finds application to exploration for oil and gas in any region but is most useful on the North American craton. Cratonic subprovinces are conveniently catalogued by age and location as basins, arches, platforms, uplifts, and shelves which have distinguishing stratigraphic and structural patterns. These patterns control the location of large fields of oil and gas pools. Typical geological controls are: (1) regional facies change as in the Clinton field in eastern Ohio or the Hugoton pool in Kansas-Oklahoma-Texas, (2) ancient offshore bars as in eastern Kansas and Oklahoma, (3) reefs, as in Utah, west Texas, and Alberta, (4) regional low-angle unconformities as in many areas of the Mid-Continent region, and (5) local structures, such as salt domes, asymmetric anticlines, faults, and folds.

Regional synthesis includes the construction and analysis of province-wide maps and sections using surface and subsurface information. These help in determining what strata in what areas in new regions are most promising for the production of petroleum.

Although already well developed commercially, the North American continent still contains many poorly known strata which may contain a wealth of undiscovered oil and gas; some of these may be: (1) Pennsylvanian and Permian strata of eastern Colorado, (2) Pennsylvanian strata of northwestern Colorado, (3) Ordovician strata of central New York, (4) Mississippian and Pennsylvanian rocks of the St. Lawrence Gulf region, (5) Mesozoic and Tertiary rocks of the Atlantic and east Gulf coastal plain, (6) rocks dating from Cambrian to Tertiary in the Arctic Archipelago (including cratonic, coastal plain, and geosynclinal strata), (7) cratonic strata especially those of Devonian age in northwestern Canada, and (8) Tertiary rocks of the Pacific coast and Alaska.

Regions 6, 7, and 8 (listed above) in northwestern North America are of special interest because of their nearly virgin status, their vast extent and their geologic similarity to major provinces in the conterminous United States and Mexico which have produced very large quantities of oil and gas. One hundred billion barrels of oil and 400 trillion cubic feet of gas have been produced in North America, primarily from the southern part. An equal volume of oil and gas may still lie undiscovered in northwestern North America.

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