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PSCarbonate Mud and Carbonate Source Rocks*
By
Paul M. (Mitch) Harris1 and Barry J. Katz2
Search and Discovery Article #40305 (2008)
Posted September 4, 2008
*Adapted from poster presentation at AAPG Annual Convention, Calgary, Alberta, June 16-19, 2005
Click to view list of articles adapted from presentations by P.M. (Mitch) Harris or by his co-workers and him at AAPG meetings from 2000 to 2008.
1 ChevronTexaco Energy Technology Company, San Ramon, CA ([email protected])
2 ChevronTexaco Energy Company, Bellaire, TX ([email protected])
Studies of modern carbonate settings have documented varied origins for mud-size sediment, including disintegration of benthic and planktic organisms, precipitation from the water column, and mechanical breakdown of larger particles. Carbonate muds accumulate in protected “low-energy” settings on the platform top and are also exported to adjacent deeper-water settings. A limitation on source-rock deposition is a result of the need for high primary productivity levels and/or elevated organic preservation. Carbonate petroleum source rocks tend to be mud/wackestones developed in two geological scenarios:
(1) Low latitude, embayments or sags where they are interbedded or underlie evaporites - Preservation can be promoted by salinity or temperature stratification which reduces diffusion of oxygen into bottom waters and near-surface pore waters. These sources are generally thin and discontinuous due to deposition in shallow, fluctuating environments. Examples of this source type are the Jurassic Hanifa Formation of the Middle East, and the Lower Cretaceous Sunniland Limestone of South Florida.
(2) Shelves during periods of rising sea level – Formation of organic-rich sediment is promoted during major transgressions, which are times of high nutrient supply and poor oceanic ventilation. Carbonate source rocks of this type are often thick and widespread. Upper Devonian – Lower Mississippian examples of this type of source rock are the Duvernay Formation of Alberta, and the Domanik facies of the Timon-Pechora, Volga-Ural, and Caspian basins. Cretaceous examples of these rocks include the Sulaiy Formation in the Middle East; La Luna and equivalents in Venezuela, Colombia, Ecuador, and Peru; and the Austin Chalk of Texas.
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Carbonate Mud Settings for Source Rocks As a result of the preservation potential of varied carbonate mud settings, most carbonate petroleum source rocks tend to be mud/wackestones developed in two geological scenarios:
Restricted, Low Latitude Embayments or Sags Preservation can be promoted by salinity or temperature stratification which reduces resupply of oxygen to the bottom waters and near-surface pore waters. These sources are generally thin and discontinuous due to deposition in shallow fluctuating environments, except where subsidence rates are sufficient to limit exposure and maintain water depths. Examples of this source type are the Permian Bone Spring Limestone of the Delaware Basin, the Jurassic Hanifa/Tuwaiq Mountain formations of the Middle East, the Lower Cretaceous Sunniland Formation of South Florida, and the Cretaceous Cobán Formation of Guatemala. The Hanifa-Tuwaiq Mountain formations (Callovian-early Kimmeridgian) were deposited within a restricted sag or intrashelf basin. Water depths in the central portion of the basin exceeded wave-base. Sediments along the basin margin tend to be organic-poor, whereas the more basinal sediments appear more typical of an oil-prone source rock. Organic carbon contents can exceed 13%, with generation potentials ranging up to ~90 mg HC/g rock. Although the organic matter varies, the more basinal sediments appear to have been originally more oil-prone. This source interval, which is cyclic in character, can exceed 170 m in thickness. There is strong evidence to support deposition under anoxic conditions. The oils derived from these sediments are sulfur-rich even when they lack evidence of biodegradation. The Cobán Formation of Guatemala is an example of a restricted embayment (Katz, 2005). Shelves during Periods of Rising Sea Level Formation of organic-rich sediment is promoted during major transgressions, which are times of high nutrient supply and poor oceanic ventilation. Carbonate source rocks of this type are often thick and widespread. Upper Devonian –Lower Mississippian examples of this type of source rock are the Duvernay Formation of Alberta, and the Domanik facies of the Timon-Pechora, Volga-Ural, and Caspian Basins. Cretaceous examples of these source rocks include the Sulaiy Formation in the Middle East; La Luna and equivalents in Venezuela, Colombia, Ecuador, and Peru; and the Austin Chalk of Texas. The Austin Chalk (Coniacian-Santonian) is an interstratified chalk and marl deposited during a major transgression. It is thought to have been deposited in water depths ranging up to ~200 m along an open shelf. Shallower water portions are typically light colored, heavily burrowed, and organic-poor, whereas deeper water portions contain increasing abundance of dark, organic-rich sediments. These darker layers are also burrowed but to a lessor degree, suggesting that deposition did not occur under anoxic conditions. Deposition within an oxic to dysoxic setting is also suggested by a number of the biomarker indices. The organic carbon contents of darker layers may exceed 20%, with generation potentials reaching almost 50 mg HC/g rock and hydrogen indices exceeding 400 mg HC/g TOC. The available data suggest a gross oil-prone source interval that may exceed 70 m. The La Luna Formation (Cenomanian-Campanian) was deposited in a similar geologic setting with one major exception. There is evidence for anoxic conditions during much of its deposition, leading to consistently higher organic carbon contents, generation potentials, and more oil-prone material.
Babcock, J.A., 1977, Calcareous algae, organic boundstones, and the genesis of the upper Capitan Limestone (Permian, Guadalupian), Guadalupe Mountains, West Texas and New Mexico, in Upper Guadalupian Facies, Permian Reef complex, Guadalupe Mountains, New Mexico and West Texas Field Conference Guidebook: SEPM Permian Basin Section, 77-16, p. 3-44. Carrigan, W.J., G.A. Cole, E.L. Colling, and P.L. Jones, 1995, Geochemistry of the Upper Jurassic Tuwaig Mountain and Hanifa formation petroleum source rock of Eastern Saudi Arabia, in Petroleum Source Rocks, p. 67-87. Katz, B., 2005, Cobàn Formation: An example of a carbonate petroleum source rock: AAPG Annual Meeting, June 19-22, 2005, AAPG Search and Discovery article #90039. King, H.H., 1948, New strikes and vast area to Permian Basin, Texas and New Mexico: World Oil, v. 128/4, p. 56-60.
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