Mesozoic basins of eastern N. America: Exploration target whose time has come

Arthur J. Pyron
Pyron Consulting
Pottstown, Pa.

Significant hydrocarbon reserves may be found in Mesozoic age rift basins of the eastern U.S. The term Mesozoic is used to describe these basins because their timing of formation ranges from the Triassic through the Jurassic.

The Mesozoic basins of eastern North America stretch from the Labrador shelf in Nova Scotia to the Florida panhandle (Fig. 1 [154,636 bytes]). These basins formed in response to extensional activity associated with the separation of Pangea in late Paleozoic-Early Mesozoic time. These rift basins apparently formed simultaneously on both the North Atlantic and Euro-African plates.

In northwestern Africa, basins with stratigraphic columns of clastic rocks (sandstones, shales, and conglomerates) similar to those in eastern North America have been documented. Similar basins formed on the South American and southern African plates in response to extensional activity concurrently with the more northern basins.¹

Significantly, hydrocarbons have been found in Mesozoic rift basins in offshore Canada (Jeanne d'Arc basin), Europe (North Sea), Northwest Africa (Gha- dames, or Central, basin), and South America (Cuyo, southern and eastern Patagonia, and Neuquen rifts). Only the rift basins found onshore in the U.S. have no identified economic hydrocarbon reservoirs.

Regional geology

In the U.S., the majority of the known Mesozoic rift basins are onshore; addi-tional basins are buried beneath Cretaceous and Cenozoic sediments of the Coastal Plain Province both onshore and offshore.

As one of the earliest features investigated by geologists visiting North America, the concept of basin stratigraphy as successive layers of clastic rocks evolved. From this perspective, these basins might not be considered prospective as hydrocarbon provinces.

In the early 1980s, a new geological interpretation of the geology of the rift basins revitalized their potential for hosting economic hydrocarbon reserves. This new interpretation identifies the formations found in the basin as lateral facies, not as layers.

Viewed this way, the geometry of the basin incorporates depositional environments conducive to generation and entrapment of hydrocarbons. It also allows better correlation to observed features in the Mesozoic basins of Algeria, Morocco, and Tunisia, and this is important, because basins in these areas have proven to host world class reservoirs, primarily of natural gas.

The depositional history of a typical onshore Mesozoic rift basin of eastern North America falls into three phases. Each phase has a unique history of deposition and lithologic type that may reflect the types of hydrocarbon traps present in these basins. A very generalized discussion of the types of geological features encountered follows.

• Phase 1 (Fig. 2 [91,674 bytes]) represents the initial phase of formation of and deposition in a typical rift basin. After formation of the rift graben and associated lateral tectonic re-adjustment, alluvial fans of coarse erosional debris formed along the upthrown block walls. Eventually, these independent alluvial fans coalesce and form a linear deposit of conglomeratic rock.

A general fining of the sediments occurs further into the basin center, with conglomerates interfingering with sandstones and siltstones. Erosion and sheet wash allow the build-up of coarser sands in immediate proximity to the alluvial fan deposits. Drainage in the basin is internal, and inter-mittent playas form.

• During Phase 2 (Fig. 3 [94,475 bytes]), tectonic growth of the basin ended or was significantly reduced. Deposits consist of reworked alluvial fan materials as well as medium coarse to fine sediments introduced by feeder streams or arroyos from outside of the rift structure. Internal drainage of the rift structure allows the formation of a lake in basin center.

Lacustrine sedimentation included fine grained organic sediments. Along the margins of the lake, vegetation was lush and included reeds, ferns, and other woody plants which as they died formed mats of decaying organic matter. The type and quality of growth was climate dependent. There were also thick growths of algae that added to the organic mix of these central lakes and helped create the organic shales observed in basin center.

As shown in the plan view, feeder streams or arroyos allow the accumulation of coarser sands in fan-like structures in immediate proximity to the lacustrine sediments. The streambeds form erratic channels, with coarse sands being isolated under siltstones and shales.

• Phase 3 (Fig. 4 [100,315 bytes]) marks the end of large scale sedimentation in the basin. Several permanent streams still deposit sediment and water into the smaller, more localized lakes, which may have been interconnected by other streambeds. Deposition of reworked sands occurs parallel to the long axis of the basin.

Late in basin formation, diabase dikes, sills, and sheets intrude along zones of previous weakness, like faults and fracture traces. Where the intruding diabase came into contact with the rift sedimentary rocks, they create a contact metamorphic chill zone of hornfels facies. The parent magma for the diabase also contributes to regional heating of the basin and aids the thermal maturation of the organic sediments.

Subsequent to Phase 3, the basin was subjected to additional post-Jurassic sedimentation, uplift and tilting, and regional erosion that created the geology currently seen. In many offshore basins, (i.e., Baltimore Canyon, the Jeanne d'Arc basin, et al.), continental deposition was followed by a period in which thick sequence of evaporites (usually halite) were precipitated. These rocks were then covered by a thick, seaward prograding layer of Cretaceous and Tertiary sedimentary rocks.

Production analogs

In attempting to draw analogies with other Mesozoic age rift basins, it is important to understand the dynamics of basin formation relative to plate tectonics.

The proto-continent of Gondwana began its breakup in Permian or early Triassic time.² The extensional activity associated with that breakup caused the formation of open rift basins along the edges of the cratons. Horst graben features formed on all of the newly formed "continents" (i.e., the North American, European, South American, and African plates).

Rift basins have historically been excellent targets for hydrocarbon exploration. Rifts allow the capture of sediments, promoting the formation of reservoirs. In addition, the internal geometry and tectonic re-adjustment of the blocks that create the graben promote the formation of structures which localize the accumulation of hydrocarbons.

An example of productive basins which are analogs to the East Coast basins are provided in Table 1 [59,248 bytes].

Van Houten³ established the geological similarity between the rift basins of eastern North America and those in northwestern Africa, especially those in Morocco and Algeria. Subsequent geological appraisal and exploratory drilling has corroborated and strengthened this relationship, including the subsurface geology of the productive area known as the Central Triassic basin (Ghadames basin).⁴

The subsurface geology of Triassic rocks in the Ghadames basin includes interbedded organic argillites, well sorted sandstone and siltstone, and shale overlying an apparent basal conglomerate and intrusive igneous rock. All sedimentary rocks of Triassic age were apparently of continental/lacustrine origin.

Hydrocarbon production in the Ghadames basin includes petroleum, condensate liquids, and natural gas.^{5 6} Several significant fields, including at least two world class reservoirs, are located in the Ghadames basin. Magliore⁷ identified Hassi Messaoud field as the pre-eminent oil field and Hassi R'Mel field as the most significant natural gas field.

Renewed interest in the hydrocarbon potential of a variety of South American basins has revealed that many of the productive basins in the southern half of the continent are half gra-bens that formed in response to the separation of the South American plate from the African plate during the Mesozoic. The Cuyo basin of Argentina is a half graben of Triassic age that has been filled with continental clastic sediments and lacustrine sediments. Subsequent to their deposition, a series of olivine basalt flows were extruded locally. A thick sequence of Cretaceous and Tertiary clastic sediments lies stratigraphically above the basalt.

Subsequent to depo-sition, motion along internal and boundary faults was reactivated by the compressional activity associated with the Andean orogeny. Dellap? et al.¹ report that lacustrine organic shales provided the source rock necessary for generation of hydrocarbons. These hydrocarbons migrated into fluvial sandstones deposited in an (internal) drainage system that ran parallel to the half graben axis.

At minimum, 15 significant hydrocarbon fields have been discovered in the basin, which has been producing for 50 years.

Since the early 1970s, explorationists have investigated the Mesozoic rift basins off Atlantic Canada for their hydrocarbon potential. The Jeanne d'Arc basin is deepest of the inter-connected Mesozoic depocenters in offshore Canada.

Sinclair et al.⁸ note that deposition in the graben began with continental red clastics of Triassic age and was followed by evaporites (salt) and shallow marine sediments of Upper Jurassic through Tertiary age. While the continental clastics are not considered the primary reservoir rocks, they are imcompletely evaluated and have been grouped by some geologists into a category of potential reservoirs.

Exploration in the Jeanne d'Arc basin began in 1971 with the discovery of thick source rock of Late Triassic-Middle Jurassic age. Subsequent exploration through the 1970s yielded wells in which flowing hydrocarbons and natural gas were recovered, including the 1979 Hibernia field discovery well tested at 20,000 b/d of oil.

Exploration companies subsequently drilled 27 wildcats that resulted in 11 discoveries. Later drilling resulted in 21 delineation wells and 8 new wildcats. Drilling in the area resulted in 15 significant hydrocarbon discoveries, 12 of which are in the Jeanne d'Arc basin (i.e., 6 oil fields, 6 natural gas fields).

Type basin discussion

Given the worldwide hydrocarbon potential of Mesozoic rift basins, there is an economic basis for further evaluation of the onshore and offshore rift basins of North America. To evaluate the hydrocarbon potential of an onshore Mesozoic rift basin, the following review is provided of the Newark basin, which has had two rank wildcat exploratory wells drilled since 1985.

The Newark basin of central New Jersey and southeastern Pennsylvania is part of a larger rift basin that incorporates the Newark, Gettysburg, and Culpeper basins and that stretches from New Jersey to Virginia.

Geological evidence suggests that significant hydrocarbon reserves exist in the Newark basin. Pratt and Burrus⁹ note that lacustrine siltstones and shales in the Newark and Connecticut basins had an average organic content of 2.0%, with the organic content for the 180 samples studied (80 samples from the Newark basin, 100 samples from the Connecticut basin) ranging from 0.5% to 6.0%.

Pratt and Burrus documented physical evidence of hydrocarbon generation and migration in these basins, including small tension fractures filled with solid bitumen, mineralized veins with intergrowths of secondary calcite and bitumen, and bitumen stained porous sandstones. Their studies of fluid inclusions in mineralized veins suggested that inclusions were trapped from heterogeneous gas saturated fluids and projected a temperature of formation of 100° C., which is consistent with other indicators of thermal maturation.

Various evidence exists of porosity and permeability in sedimentary rocks in the Newark basin. A report¹⁰ on the work of Lamont-Doherty Geological Observatory (LDGO) indicated that the Newark basin has possible gas potential. Lamont also noted that the basin has thick sequences of (organic) shales (which could be adequate source rock), and sandstones with good porosity. The elements required for formation of a hydrocarbon reservoir (i.e., source rock, porosity, and permeability) are present in the Newark basin (Fig. 5 [56,665 bytes]).

One of the problems in the Newark basin is that exposures of component rocks are limited by cover, both vegetative and cultural. Know- ledge of the subsurface is limited; surface geology alone will not provide the answers necessary to identify productive structures.

Alternate exploration methods like remote sensing, geochemistry, or nonseismic geophysics, will be of great value in identifying hydrocarbon reserves in stratigraphic-diagenetic traps. Additional subsurface control, either by exploratory wildcats or stratigraphic tests will provide additional support in targeting potential reservoirs. Three exploration wells have been drilled in the basin (Table 2 [45,882 bytes]) with mixed results.

North Central Oil Corp., Houston, drilled two of these wells in the mid-1980s. Electric logs for both wells remain proprietary, but the northern well, North Central 1 KCI Cabot, apparently encountered a show of gas in the organic shale section that was economic and comparable to a Devonian shale well in the Appalachian basin. The second well was drilled farther west in the contact metamorphic zone of a diabase intrusion and did not report a show of hydrocarbon.

Both of the North Central wells were based on prospects developed by use of seismic prospecting methods.

Interestingly, the 1 Cabot KCI well offset by some 200 ft a well drilled in the late 1800s by the Eastern Oil Co. of Philadelphia. This well, drilled to about 2,100 ft by cable tools, apparently uncovered an 8 ft thick coal seam and two shows of live petroleum.

The well record, researched by a contemporary geologist, J.P. Lesley,¹¹ and by Dean McLaughlin,¹² indicates that the well encountered salt water (i.e., brackish water) about 1,500 ft below surface. If this report is true (neither of the previous investigators believed that it was), then this is the first reported case of anything other than potable water being found in the basin.

Given the results of these three test wells, additional investigation appears war-ranted.

The author of this article completed a regional study of the Newark basin¹³ in the early 1990s. Using proprietary and well known remote sensing interpretation methods, he identified several areas within the basin in which there was a coincidence of lineaments and tonal anomalies (Fig. 6 [123,038 bytes]).

The coincidence of lineaments and tonal anomalies has proven to be a statistically valid indicator and accurate exploration tool for identifying reservoirs and new exploration targets in previously underdeveloped basins.¹⁴ Their presence provides the author with a level of confidence in appraising the hydrocarbon potential of a basin.

Based on the 1993 study and subsequent ground truthing, several areas have been identified that might merit evaluation by drilling. This is obviously the next step in the exploration process.

Conclusion

Synthesis of published geological data, Landsat interpretation, and corollaries with analog fields in productive rift basins worldwide, suggests that the Newark basin is prospective for hydrocarbons.

This is important from a strategic viewpoint, because in most cases, two nonproductive wells might condemn a rank wildcat area like the Newark superbasin. However, application the lateral facies concept to the basins allows the theoretical assumption that reservoirs with hydrocarbon may remain.

One detrimental factor in evaluating the Newark basin is the lack of evaporite seal rocks over potential reservoirs. The absence of a cap rock over reservoirs is a significant concern but not catastrophic to the search for hydrocarbons. Instead of traditional structural trapping, reservoirs may take the form of lateral diagenetic or stratigraphic traps. Identification of these types of traps may require unique exploration methods, including the deliberate search for the subtle trap.

While the Newark basin is offered in this paper as a type basin relative to exploration modeling, each rift basin has unique geology and a unique data set. It will be important for explorationists to apply ingenuity to the respective exploration problem.

Any company envisioning such a search must be well capitalized, overly optimistic, and sensitive to environmental regulations and political reality. Given the potential for reward and the available access to market (the region is home to more than 60% of the U.S. population), the risk may be warranted.

References

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Hallam, A., Mesozoic geology and opening of the North Atlantic, Journal of Geology, Vol. 79, No. 2, 1971, pp. 129-157.
Van Houten, F.B., 1977, Triassic-Liassic deposits of Morocco and eastern North America: Comparison, AAPG Bull., Vol. 61, 1977, pp. 79-99.
Ali, Odeh, Stratigraphy of the Lower Triassic sandstone of Northwest Algerian Sahara, Algeria, AAPG Bull., Vol. 57, No. 3, 1973, pp. 528-540.
U.S. Department of Energy, Libya, Algeria, and Egypt-Crude oil potential from known deposits, Foreign energy supply assessment program series, U.S. DOE, DOE/EIA-0338, April 1982, 105 p.
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Lesley, J.P., An important boring through 2,000 feet of Triassic in eastern Pennsylvania, American Philosophical Society, Proceedings Vol. 29, 1891, pp. 20-25.
McLaughlin, D.B., The Revere well and Triassic stratigraphy, Pa. Acad. of Sciences Proceed. Vol. 17, 1943, pp. 104-110.
Pyron, A.J., Use of space based detectors as a strategic tool to evaluate the hydrocarbon potential of the U.S., National Aeronautics and Space Administration Visiting Investigator Program, John C. Stennis Space Center, Stennis Space Center, Bay St. Louis, Miss., 1993.
Pyron, A.J., Tonal indicators of fractured reservoirs: A remote sensing case study in the San Juan basin, northwestern New Mexico, Symposium on Natural Fracture Systems, Four Corners Geological Society, 1997, pp. 105-116.