WORLD PETROLEUM SYSTEMS WITH JURASSIC SOURCE ROCKS

H. Douglas Klemme
Geo Basins Ltd.
Bondville, Vt.

Fourteen petroleum systems with Upper Jurassic source rocks contain one quarter of the world's discovered oil and gas.

Eleven other systems with Lower and Middle Jurassic source rocks presently have a minor but significant amount of discovered oil and gas.

The purpose of this article is to review the systems geologically, describe their location in space and time on a continental scale, estimate their relative petroleum system recovery efficiencies, and outline the effect their essential elements and processes have on their "petroleum plumbing."

Difficulty in labeling Upper Jurassic petroleum systems by major source and major reservoir rock was encountered in:

Regions of the world where there is a lack of stratigraphic names requiring that the age of the rock units be used for source and reservoir;
Petroleum systems so large and extensive that several stratigraphic names have been applied to the same source rock; and
Many Upper Jurassic systems where a great deal of upward petroleum leakage occurred so that many (multiple) reservoirs rather than one main reservoir, contain the bulk of the accumulated petroleum.

The global scale of this study requires that Petroleum systems be lumped for brevity of discussion and the lack of detailed information. It is apparent that as more attention is given to mapping each pod of active source rock and its related hydrocarbons, the 25 petroleum systems described here will probably increase in number.

However, as global as this approach may be, it demonstrates the usefulness of identifying all those petroleum systems in the world that include one source rock interval.

U. JURASSIC SOURCES

About 80% of the recoverable barrels of oil equivalent (BOE) generated from Upper Jurassic source rocks comes from sedimentary basins or provinces located on or in the interior of continents. The other 20% comes from basins or provinces along continental coastal areas.

The interior basins or provinces contain about 70% of the Upper Jurassic mature source rock area and 56% of the volume. Continental coastal areas contain 30% of the Upper Jurassic mature source rock area and 44% of the volume.

If the source rock richness is similar in both provinces, then by material balance the continental interior would appear to contain the more efficient petroleum systems.

Upper Jurassic source rocks were deposited on both Precambrian cratons and on their surrounding Phanerozoic accretionary terranes.

More than 50% of the BOE from Upper Jurassic source rocks comes from those source rocks deposited on the Precambrian craton, mainly in North Gondwana, along the south Tethyan margin. These areas include the Arabian/Iranian basin, Yemen rift province, Papua basin, and Australian Northwest shelf.

The remaining BOE was deposited, about equally, over Hercynian and Caledonian accreted terranes around and on Laurasia.

The eastern West Siberia basin and Northwest European shelf (northern North Sea and Norwegian shelf) have Upper Jurassic source rock deposited over Caledonian age accreted/orogenic terranes. The Gulf of Mexico, the Middle Caspian, Amu Darya, and Jeanne d'Arc provinces, Scotian shelf, western West Siberia, and Vienna basins overlie Hercynian accreted terranes.

PETROLEUM REALMS

Petroleum realms consist of the Tethyan realm, which divides the South Gondwana realm from the Boreal realm, and the Pacific realm.

The Tethyan realm is related to an east-west equatorial seaway that opened and closed during the Hercynian, Kimmerian, and Alpine orogenic events while micro-plates were rift/drifted from northern Gondwana and collided/docked on the southern accretionary margin of Laurasia. South Gondwana and Boreal realms represent the interior or continental portions of Gondwana and Laurasia.

The Pacific realm was formed by the Hercynian circum-Pangea accretionary margin followed by the Kimmerian to Holocene circum-Pacific accretionary-orogenic zone.

Similar to the BOE recovery averages for most all the world's source rocks, the Upper Jurassic Tethyan realm appears to have Petroleum systems with the greatest recovery per unit area of mature source rock, while the Jurassic source rock of the Boreal realm indicates a BOE recovery per unit volume similar to an average of world total source rock recovery.

STRUCTURAL FORM

Structural form reflects the evolutionary tectonic stage of various basin types.

Nearly all Upper Jurassic source rocks were deposited in the structural form of the rift-sag cycle of a basin's tectonic evolution, with 10% of the BOE coming from the source rock deposited in the rift stage and nearly 90% from the sag stage. A minor amount of the BOE (Scotian shelf) was from source rocks deposited in the prograding sequence into a half sag.

Other major world source rocks include the Silurian source rocks deposited only on the platform structural form or no rift-sag, the Upper Devonian source rocks with only 12% deposited on the rift-sag of Laurasia, and the Pennsylvanian-Permian source rocks with 47% deposited on the rift-sag structural forms of Laurasia.

In contrast the mid-Cretaceous involves the deposition of 93% of their source rocks on rift-sag structural forms located in Laurasia and North Gondwana.

It is likely that the absence of the rift-sag as a depositional structural form in the Silurian source rocks and its limited presence in Devonian source rocks are due to both Caledonian and Hercynian destruction of early and middle Phanerozoic source rocks deposited in the structural forms of the rift-sag cycle and to the progressive biologic evolution from shallower water to deeper and deeper water ecologic niches as the habitat of bottom scavengers evolves.

Most of the Upper Jurassic source rocks occur stratigraphically very close to the interface of the rift as it changes to a sag.

JUXTAPOSITION OF ROCKS

In the four largest petroleum systems, the recovery efficiency is directly related to the proximity of the Upper Jurassic source rock to the charged reservoir rocks and traps and indirectly to the leakage of petroleum around or through the cap rock.

The supergiant and giant petroleum systems to be compared are:

The Hanifa-Arab system located in the Arabian/Iranian basin.
The Bazhenov-Neocomian system in the West Siberia basin.
The Kimmeridgian "hot shale"-Brent system in the Northwest European shelf; and
The Tamman/Smackover system in the Gulf of Mexico province.

In these petroleum systems, their prolific nature is related to where the high percentage of the system's BOE is trapped relative to the active source rock. Here the reservoir rock is immediately capped or sealed by either an especially efficient cap rock or the "source and the seal" for the oil (such as the efficient Hith anhydrite in the Hanifa-Arab and "source-seal" Kimmeridgian "hot shales" in the Kimmeridgian "Hot shale"-Brent systems).

Conversely, petroleum systems that include poor seals have about 80% of the BOE in reservoir rocks well above the active source rock. Lacking efficient caprock or seal, oil "leaks" upwards to overlying reservoir rocks and structural sequences, which results in a less efficient or prolific petroleum system, as in the Gulf of Mexico and West Siberia.

L. M. JURASSIC

Lower and Middle Jurassic sedimentary rocks occupy essentially the same position and underlie much of the Upper Jurassic.

At approximately 30 locations there exists Lower and Middle Jurassic source rock that results in some hydrocarbon production. This represents only 5% of the total production from all Jurassic source rocks.

Two thirds of the area covered by Lower and Middle Jurassic source rocks is overlain by an Upper Jurassic source rock, while one third of the Lower and Middle Jurassic source rocks lack an overlying Upper Jurassic source rock.

Where an Upper Jurassic source rock is absent but where production came from Lower and Middle Jurassic source rocks, two thirds of these hydrocarbon occurrences are related to regional or local uplift. This uplift is a departure from the "Vail sea level curve" which rises during the Upper Jurassic because this is the early initiation of the worldwide Neocomian regressive, clastic depositional event. These regressive sequences most often occur stratigraphically above the Mixed and Humic organic facies.

The other one third of the hydrocarbon occurrences occur above the Tethyan facies where either the open marine carbonate facies or the deep marine "starved basin" facies were developed but never developed an inter-shelf depression, either in or on the open marine shelf, for deposition of a laminated lime mudstone into an anoxic environment to form bituminous source rock.

L. CRETACEOUS

In certain areas, Jurassic source rocks can extend up into the Lower Cretaceous.

Lower Cretaceous (Berriasian to Barremian) deltaic to open marine sedimentary rocks were deposited in 30 areas where production was generated from Lower and Middle Jurassic source rocks and in 20 areas where production originated from Upper Jurassic source rocks.

Where sediments were derived from the craton or continental interior, they were deposited as a regressive, deltaic sequence that grades to open marine. These sequences usually occur in the foldbelt and foreland basins as well as rifted basins. However, there is a tendency for more open marine sediments to occur in the divergent margin basins.

Generally the Neocomian regressive phase contains more reservoir rocks than source rocks, while the transgressive (rising sea level) Cretaceous (Aptian to Turonian) includes worldwide source rocks that have generated even more BOE than the Upper Jurassic (30% or more of the world's discovered BOE).

RESERVOIR AGE, DYNAMICS

Using the distribution and volume of conventionally recoverable BOE, several observations can be made about petroleum systems with an Upper Jurassic source rock.

Most of the reservoir rocks are Jurassic (63%) in age followed by Cretaceous (33%) and Tertiary (3%), and their lithology is more likely to be carbonate (60%) rather than siliciclastic sandstone (40%).

The onset of oil generation is usually Late Cretaceous-Early Tertiary (76%) as opposed to Late Jurassic-Early Cretaceous (17%) or Late Tertiary (7%). Most trap growth occurred before the onset of oil Generation (90%) as compared to during (5%) and after (5%) oil generation.

RECOVERY EFFICIENCY

Petroleum system recovery efficiency is the percent of ultimately conventionally recoverable BOE (1 bbl of oil is energy equivalent to 6,000 cu ft of gas) to the amount of BOE that could have been generated from a pod of active source rock if the source rock were completely spent.

This recovery efficiency requires the estimator to make a best guess at the ultimate, conventionally recoverable BOE and to assume that the entire volume of source rock is spent, which in most cases is untrue.

The petroleum system recovery efficiency is the percent of ultimately recoverable BOE that could be generated from a spent source rock.

Estimates of petroleum system recovery efficiency reveal that in Upper Jurassic petroleum systems 0.86% to 0.10% of the hydrocarbon is recoverable from the spent source rock. Said another way, for every 100 BOE generated only 6.86 BOE is recoverable.

It is estimated that the world's petroleum systems have a range of recovery efficiency from 4% to 0.04% (8% to 0.62% range when doubled and halved by some authorities).

For example, within the Hanifa-Arab petroleum system in the Arabian/Iranian basin, the southernmost mature source area-Southwest Arabian Gulf and Southeast Saudi Arabia-when calculated as a separate unit has a recovery efficiency of 2.3%.

The entire Gulf of Mexico's Upper Jurassic petroleum system, the Tamman/Smackover, has a calculated efficiency of 0.46%. When calculated for the area within the U.S., excluding the Mexican Tampico-Chicontopec-Reforma-Campeche provinces), the recovery efficiency is only 0.18%.

Variability in recovery efficiency occurs not only between petroleum systems but within portions of the same system.

'OTHER' PLUMBING FACTORS

Other plumbing factors include the principle plumbing ingredients of reservoir and cap rock, trap, and dynamics (juxtaposition and trap formation-maturation/migration timing).

When other plumbing factors are essentially similar, greater mature source rock volumes result in higher petroleum accumulation.

However, the ultimate per unit recovery of petroleum (richness) and the relative magnitude of the petroleum system recovery efficiency are not necessarily related directly to the magnitude of petroleum available from the pod of spent source rock.

Once active source rock is present, on average, higher petroleum system recovery efficiency is noted in those systems where the reservoir and cap rocks are of high quality and extend over a large area, where the size of traps and where the dynamics and timing of these "other" plumbing ingredients are of high rank.

In examples discussed in this complete work but omitted from this article for brevity, the influence of these "other" plumbing ingredients are as important if not more important than the source rock quality or the amount of available petroleum from the system's active source rock.

In several instances, both a higher recovery efficiency and higher recoverable petroleum per unit area are noted in systems with less available petroleum per unit area from the mature source rock than in petroleum systems with more available petroleum in their active source rock. These variations in recovery efficiency are also related to the geometry of the source "pod" within the petroleum system.

ACKNOWLEDGMENTS

The author acknowledges and thanks I. Maycock, R. Church, and M. Nemic for aid in data assembly, L.B. Magoon for instructive discussions and considerable aid in preparation of the data, and R. Johnson for drafting.