Albert B. Dickas

A possible southeastern extension of the Midcontinent Rift System located in Ohio

In 1988 a stratigraphic test core drilled in southwestern Ohio penetrated a previously unknown structure and sedimentologic unit. Core analyses disclosed a sequence of lithic arenite and siltstone of probable pre-Phanerozoic age, deposited within an alluvial-fluvial environment. A seismic reflection profile across the core site shows a sequence of strong, horizontal Paleozoic reflectors unconformably overlying eastward dipping, layered units of poor reflectivity. Beyond the core hole total depth, a regime of excellent reflectors is seen. The pre-Mount Simon (Upper Cambrian) sedimentary sequence encountered has been defined as the type section of the Middle Run Formation. Point counter analyses of typical Middle Run Formation samples indicate that this sequence is similar in composition to middle Proterozoic units associated with the Midcontinent Rift System in the Lake Superior region. The newly discovered structure in Ohio is also similar to half-graben structures composing the Lake Superior Basin portion of the Midcontinent rift. On the basis of similarities in lithology, stratigraphy, structure, and proximity to regional gravity anomalies, the Middle Run Formation and its apparent half-graben basin is proposed as evidence for the probable extension of the Midcontinent Rift System southward, from its normally accepted termination in southeastern Michigan, into southwestern Ohio.

The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region

Abstract The Midcontinent Rift System (MRS) of central North America is a 1.1-Ga, 2500-km long structural feature that has been interpreted as a triple-junction rift developed over a mantle plume. As much as 20 km of subaerial lava flows, mainly flood basalts, are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds. This rock sequence, known as the Keweenawan Supergroup, has been penetrated by a few deep boreholes in the search for petroleum. In this paper, two deep boreholes in the Upper Peninsula of Michigan are described in detail for the first time. Both the Amoco Production #1-29R test, herein referred to as the St. Amour well, and the nearby Hickey Creek well drilled by Cleveland Cliffs Mining Services, were 100% cored. The former is 7238 ft (2410 m) deep and the latter is 5345 ft (1780 m) deep. The entirety of the stratigraphic succession of the Hickey Creek core correlates very well with the upper portion of the St. Amour core, as determined by core description and point-counting of 43 thin sections selected out of 100 studied thin sections. Two Lower Paleozoic units and two Keweenawan red bed units—the Jacobsville Sandstone and the underlying Freda Sandstone—are described. The Jacobsville is largely a feldspatholithic sandstone and the Freda is largely a lithofeldspathic sandstone. Below the Freda, the remaining footage of the St. Amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate, sandstone, siltstone, and shale; black shale; individual basalt flows; and a basal ignimbritic rhyolite. This lower portion of the St. Amour core presents an enigma, as it correlates very poorly with other key boreholes located to the west and southwest. While a black shale sequence is similar to the petroleum-bearing Nonesuch Formation farther west, there is no conglomerate unit to correlate with the Copper Harbor Conglomerate. Other key boreholes are distributed over a 1300-km distance along the better known southwest arm of the triple-junction MRS, and can be correlated rather well with the units that are exposed in the Lake Superior region. However, a definitive explanation of the anomalous, deeper St. Amour stratigraphy is elusive and any explanation is tenuous. A possible explanation for this anomalous stratigraphy may be the geographic proximity of the St. Amour borehole to the Keweenawan Hot Spot (mantle plume), the suggested thermal force behind the development of the MRS. Similarly, a drastic change in structural architecture may be explained by this geographic relationship. Thus, within the locale of this rifting center, complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm.

The Association of Hydrocarbon with Basement Rock of Pre-Phanerozoic Age

Until recently, the occurrence of oil and gas in rock of pre-Phanerozoic age was considered of insignificant importance, as demonstrated by published reports and the supposed lack of hydrocarbon source and reservoir rock of Precambrian age. Discoveries since 1956 of economic hydrocarbon deposits associated with Precambrian basement rocks have led to world-wide exploration for similar resources.

Dengying Formation Gas System of the Sichuan Basin, Southwest China: Model for Precambrian Indigenous Hydrocarbon Accumulation

The Dengying Formation (Late Sinian age) gas system encompasses the Weiyuan gas field and other gas-producing structures. The natural gas included in this system is both sourced and reservoired in rocks of the Dengying Formation of Late Sinian age. The source rocks, dominated by micrite and dolomite, are sapropelic and have evolved in most regions to the overmature, dry gas stage. The peak of the natural gas generation occurred at the end of the Jurassic. The Sinian age reservoir rocks are formed of cavern-porous, fractured porosity, with both the primary and secondary porosities controlled by lithofacies distribution, unconformity surface development, and fracturing. The seal is provided by lower Cambrian shale. The Leshan-Longnusi Caledonian paleohigh, located near and partly within the hydrocarbon-generating center, marks this paleo-uplift as a most opportune site for the future discovery of commercial Sinian natural gas reserves.

Structure of the Pripyat-Dnieper-Donets Rift System and Affiliated Economic Geology

The southern half of the East European Platform is bisected by an intracratonic, 1500 km long, extensional feature which was structurally active from Devonian into Permian time. The Pripyat-Dnieper-Donets rift system is composed of three basins of approximately equal area and differing tectonic development. This NW-SE trending rift system overlies north-trending aulacogens of Late Proterozoic age, causing Middle and Upper Paleozoic rocks to locally overlie strata of Riphean age unconformably.

Structural Geometry of the Superior Zone, Midcontinent Rift System

The 1.1 Ga Middle Proterozoic Midcontinent Rift System (MRS) is a major intracontinental, thermo-tectonic structure that is traced by gravity and magnetic data, subsurface drilling, and outcrop control over a length of 2,000 km within the central United States (Van Schmus and Hinze 1985; Cannon et al. 1989) (Fig. I). Outcrop, known only in the Lake Superior region, in conjunction with geopotential data, defines a polyphase tectonic basin filled principally by plateau lava and sedimentary rock comprising the Keweenawan Supergroup. Basalt, and associated plutonic rocks, underlain by thin quartz sandstone and interbedded with thin, clastic sedimentary units of local extent, was extruded during an early extension phase of rift development, occurring approximately 1108–1094 Ma (Paces and Davis 1988). As igneous activity waned, a suite of upward maturing clastic strata was deposited. Finally, the tectonic focus changed from extension to compression, and a final phase of clastic sedimentation occurred.

The Occurrences of Descendant and Indigenous Hydrocarbon in Precambrian Rocks of China

Hydrocarbon stored in Precambrian rocks is widespread in China. Hydrocarbon-bearing units are mainly associated with metamorphic and carbonate rocks of Archean and Middle-Late Proterozoic (including Sinian) age. Reservoirs are characterized by secondary porosity associated with karstification, fracturing, buried dissolution, and buried recrystallization. Hydrocarbon originated from both Phanerozoic and Precambrian source rocks, so descendant and indigenous hydrocarbon accumulation can be categorized by relation of source to reservoir rock. By volume, descendant hydrocarbon dominates.