Graptolite Shale - Worldwide Distributed Ordovician and Silurian Source Rock

Agapitov, I. (Lomonosov Moscow State University)

OnePetro 

Abstract

Graptolite shales are a type of fossil shales that contain a large number of graptolite imprints and remains. These deposits are characterized by high TOC contents (Corg = 2–18%). Based on the data of many studies, graptolite shales are one of the main hydrocarbon sources that formed oil and gas fields in Paleozoic deposits around the world, e.g., the Silurian graptolite shales make up to 9–15% of all hydrocarbons that form the oil and gas fields in the largest petroleum basins.

The main accumulation areas of graptolite shales during the Ordovician were the margins of Baltica, Laurentian, and Gondwana (Yapetus Ocean shelf). One of the belts (northern) extended from the southern periphery of the Baltic Shield to the Appalachian Basin; another (southern) belt stretched from the sedimentary basins of the African–Arabian margin of Gondwana to the shelf and continental slope of southern Laurentia, the present-day Western Interior Coal Region, Permian basin, and Ouachita belt. In the Silurian, the southern (northern Gondwanan) belt began

to play the leading role, while another northern belt incorporated the southern regions of Baltica and the microcontinents approaching to it (Armorica, Perunica, and Iberica). In the sedimentary basins of Laurentia, Silurian graptolite shales were not widespread; they are reported only for east Greenland and in the Michigan basin. The contribution of Paleozoic graptolite shales into

the generation of hydrocarbons that formed multiple oil and gas fields worldwide is quite high. Based on the data of G. Ulmishek and H. Klemme (1991), oil-and-gas source rocks of Ordovician and Silurian ages, which are mostly graptolite shales, produced up to 9% of all the hydrocarbon reserves discovered by the late 20th century. For particular regions, this contribution was even more. It is believed that Silurian graptolite shales produced 80 to 90% of the hydrocarbons stored in giant oil field of North Africa. The Silurian graptolite shales are also thought to have played the main role in the formation of the South Pars/North Field gas condensate field in the Persian Gulf, which is the largest field for natural gas in the world.

During work with more than 41 publications schemes of distribution of graptolitic shales in Ordovician and Silurian was composed. Such knowledge about source rock is one of the many steps to Petroliferous basins insight.

Keywords: graptolite shales, Silurian, Ordovician, Yapetus, Rheic Ocean

Introduction

Graptolites, or the Graptolithina class (Greek graptos: painted, drawn; lithes: stone) are referred to the Hemichordata type. In contrast to true Chordata, this is expressed not by a long tenia, but a small dorsal apophysis of the intestine in the pharynx zone. Graptolite remains are preserved in the form of small tubes that are united into colonies of up to 10 cm in size. These pipes (thecae) are of chitine-like appearance and of scleroprotein (condensed protein polymers), not chitine (carbohydrate polymers) composition, as was previously believed. The cells are up to 1 mm in cross size and up to 4 mm long; they are of cylindrical, conic, beak_like, or hook-like shapes. The first theca of a colony, which is called a sicula (Greek sicula; small dagger) is narrow–conic in shape. Graptolites are marine organisms that lived in the seawater of normal salinity according to the way of life of benthic, plankltonic, and pseudoplanktonic animals. “Siculae of benthic and pseudoplanktonic colonies have a thread-like sprout or basal plate, with which a colony is attached to the bottom or any floating objects. Some planktonic graptolite colonies contain air bubbles” (Mikhailova and Bondarenko, 2006). 

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