Effective Petroleum Source Rocks of the World: Stratigraphic Distribution and Controlling Depositional Factors

H.D. Klemme and G.F. Ulmishek

Search and Discovery Article #30003 (1999)

(Condensed from their article published in the AAPG Bulletin, v. 75, 1991, p. 1809-1851; reprinted in part and adapted for online presentation.

More than 90% of original recoverable oil and gas reserves in the world has been generated from source rocks of six stratigraphic intervals--(1) Silurian (generated 9% of the world's reserves), (2) Upper Devonian-Tournaisian (8% of reserves), (3) Pennsylvanian-Lower Permian (8% of reserves), (4) Upper Jurassic (25% of reserves), (5) middle Cretaceous (29% of reserves), and (6) Oligocene-Miocene (12.5% of reserves) (Fig. 1). Facies and structural form maps (Animation, or Time-Lapse Sequence, A) for the six intervals are shown in quasi-animation form, as are corresponding petroleum source rock maps (Animation, or Time-Lapse Sequence, B). Together, these intervals represent only one-third of Phanerozoic time. The concentration of source rocks in these several stratigraphic intervals demonstrates uneven distribution of source rocks in time, and obviously it does not result from a single mechanism because the areal distribution, localization in specific structural forms, and even the geochemical character of source rocks changed from one interval to another. Key features of source rocks of these intervals in basins of development are summarized in Tables 1-6. The primary factors that controlled the areal distribution of source rocks, their geochemical type, and their effectiveness are geologic age, paleolatitude of the depositional areas, structural forms in which the deposition of source rocks occurred, and the evolution of biota. Silurian, Upper Devonian-Tournaisian, Upper Jurassic, and middle Cretaceous source rocks were deposited during generally transgressive stages. In contrast, the deposition of Pennsylvanian-Lower Permian and Oligocene-Miocene source rocks took place during marine regressions. Favorable combinations of a number of factors--tectonic, climatic, hydrologic, and biologic--resulted in high effectiveness of source rocks of the six principal intervals. Thus, the intermittent appearance of widespread effective source rocks was not caused by any cyclic geologic process.

The most important change in the character of source rocks during the Phanerozoic was the appearance and expansion of source rocks containing type III kerogen and coal. The effectiveness of these source rocks also grew, reaching its maximum in the Oligocene-Miocene. However, the areal extent and effectiveness of source rocks with type II kerogen, primarily black shale facies, decreased during the same time. Low latitudes were more favorable for deposition of source rocks with kerogen types I and II. In contrast, more source rocks containing kerogen type III and coal were deposited in high latitudes (except for the Oligocene-Miocene interval). A very high effectiveness of source rocks with kerogen types I and II deposited in low latitudes is connected with the widespread presence of carbonate reservoir rocks and evaporite seals that helped trap and retain petroleum.

Platforms open to the oceans controlled the deposition of effective source rocks during the early and middle Paleozoic. In the Mesozoic, the main source rock deposition shifted to the silled basins located in circular and linear sags. In the Pennsylvanian-Early Permian, rifts and foredeeps controlled the principal deposition of petroleum source rocks. In the Oligocene-Miocene, the source rock deposition occurred in these same two structural forms and also in deltas and half sags.

We suggest that biologic evolution played an important role in the history of source rock deposition. Different groups of producers evolved and colonized new ecologic niches that expanded areas of bioproduction. Source rocks with terrestrial organic matter appeared. However, the contemporaneous evolution of consumers and decomposers resulted in the decrease of variety of conditions suitable for preservation of organic matter with type II kerogen. The areal expansion of organic matter production and the simultaneous attenuation of conditions of its preservation in the ocean resulted in a change from source rocks with exclusively marine organic matter in the early Paleozoic to source rocks with dominant terrestrial organic matter in the Tertiary.

The maturation rate of source rocks, similar to their rate of deposition, was also quite uneven in geologic time. The major stage of maturation of Paleozoic source rocks was associated with the Hercynian orogeny and widespread deposition of thick molasses. The next principal maturation stage occurred during the Late Cretaceous and Tertiary in connection with the Alpine orogeny and deposition of thick clastic wedges of this age.

Figure 2 summarizes deposition of source rocks, their maturation, and entrapment. As would be expected, the line of effective source rocks deposition is mostly located above the line of trapped petroleum reflecting the dominance of upward-directed vertical migration; however, the relative position of these lines is reversed at the base of the Upper Jurassic. This means that all effective source rocks deposited before the Late Jurassic could not have provided all of the oil and gas trapped in the sub-Upper Jurassic section. This relationship emphasizes the importance of the downward migration of petroleum.

The cumulative maturation line demonstrates the generally young age of most of the world's discovered oil and gas. Almost 70% of the world's oil and gas reserves was generated since the Coniacian, and nearly 50% of the reserves was generated and trapped since the Oligocene. About 6% of the world's petroleum reserves is biogenic gas from still-immature source rocks; the bulk of this gas occurs in northern West Siberia.

The high effectiveness of source rocks with type II kerogen that were deposited in low latitudes resulted in rich oil and gas reserves of the Tethyan realm, a Silurian-Holocene latitudinal seaway between Gondwana and the northern group of continents (Fig.3). On the north, the realm is bounded by the Hercynian collision zone and includes foreland basins genetically connected with Tethyan tectonics. In addition to clastic rocks, the realm contains widespread carbonate reservoirs and evaporite seals. The source rock deposition was aided by the successive opening and collisional closing of proto-Tethys, paleo-Tethys, and neo-Tethys that developed rift/sag structural forms favorable for the formation of silled basins. The Tethyan basins are developed over less than one-fifth of the world's land area and continental shelves, yet they contain over two-thirds of the original petroleum reserves.

The Boreal realm is second in richness. Its petroleum reserves are in Paleozoic carbonate and clastic reservoirs and in Mesozoic clastic reservoirs. Paleozoic petroleum primarily is connected with Upper Devonian-Tournaisian source rocks deposited on platforms in low paleolatitudes. Most Mesozoic petroleum was generated from Upper Jurassic and middle Cretaceous source rocks deposited in sags over rifts.

The Pacific and south Gondwana realms are relatively poor in oil and gas. The principal petroleum reserves of the Pacific realm are in Tertiary rifts and deltas of the active margin and in foredeep basins connected with orogenies of the eastern Pacific rim. In the south Gondwana realm, major petroleum reserves are confined to Mesozoic-early Tertiary rifted passive margin basins and to late Tertiary deltas. Much of the petroleum in both realms is high-wax oils resulting from either type I or type III kerogen.

Figure 1 - Stratigraphic distribution of effective source rocks given as a percentage of the world's original petroleum reserves generated by these rocks. (Fig. 1 in Klemme and Ulmishek, 1991).

Figure 2 - Cumulative chart of effective source rock deposition, source rock maturation, and petroleum trapped in the stratigraphic succession given as a percentage of world's original petroleum reserves. (Fig. 28 in Klemme and Ulmishek, 1991).

Figure 3 - Petroleum realms of the world. (Fig. 29 in Klemme and Ulmishek, 1991).

Animation, or Time-Lapse Sequence, A

Animation, or Time-Lapse Sequence, B

Tables

Major Productive Basins Having Silurian Source Rocks (part of Table 1 in Klemme and Ulmishek, 1991; please refer to it for citations).

Major Productive Basins Having Upper Devonian-Tournaisian Source Rocks (part of Table 1 in Klemme and Ulmishek, 1991; please refer to it for citations).

Major Productive Basins Having Pennsylvanian-Lower Permian Source Rocks (part of Table 1 in Klemme and Ulmishek, 1991; please refer to it for citations).

Major Productive Basins Having Upper Jurassic Source Rocks (part of Table 1 in Klemme and Ulmishek, 1991; please refer to it for citations).

Major Productive Basins Having Middle Cretaceous Source Rocks (part of Table 1 in Klemme and Ulmishek, 1991; please refer to it for citations).

Major Productive Basins Having Oligocene-Miocene Source Rocks (part of Table 1 in Klemme and Ulmishek, 1991; please refer to it for citations).

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