Abstract
There are rich oil and gas resources in marine carbonate strata worldwide. Although most of the oil and gas reserves discovered so far are mainly distributed in Mesozoic, Cenozoic, and upper Paleozoic strata, oil and gas exploration in the Proterozoic–Lower Paleozoic (PLP) strata—the oldest marine strata—has been very limited. To more clearly understand the oil and gas formation conditions and distributions in the PLP marine carbonate strata, we analyzed and characterized the petroleum geological conditions, oil and gas reservoir types, and their distributions in thirteen giant oil and gas fields worldwide. This study reveals the main factors controlling their formation and distribution. Our analyses show that the source rocks for these giant oil and gas fields are mainly shale with a great abundance of type I–II organic matter and a high thermal evolution extent. The reservoirs are mainly gas reservoirs, and the reservoir rocks are dominated by dolomite. The reservoir types are mainly karst and reef–shoal bodies with well-developed dissolved pores and cavities, intercrystalline pores, and fractures. These reservoirs are highly heterogeneous. The burial depth of the reservoirs is highly variable and somewhat negatively correlated to the porosity. The cap rocks are mainly thick evaporites and shales, with the thickness of the cap rocks positively correlated to the oil and gas reserves. The development of high-quality evaporite cap rock is highly favorable for oil and gas preservation. We identified four hydrocarbon generation models, and that the major source rocks have undergone a long period of burial and thermal evolution and are characterized by early and long periods of hydrocarbon generation. These giant oil and gas fields have diverse types of reservoirs and are mainly distributed in paleo-uplifts, slope zones, and platform margin reef-shoal bodies. The main factors that control their formation and distribution were identified, enabling the prediction of new favorable areas for oil and gas exploration.
Highlights
Oil and gas fields with recoverable reserves of more than 500 9 106 bbl of oil equivalent are referred to as giant oil and gas field (Halbouty 2003; Bai 2006)
Most of the oil and gas reserves discovered so far are mainly distributed in Mesozoic, Cenozoic, and upper Paleozoic strata, oil and gas exploration in the Proterozoic–Lower Paleozoic (PLP) strata—the oldest marine strata—has been very limited
Our analyses show that the source rocks for these giant oil and gas fields are mainly shale with a great abundance of type I–II organic matter and a high thermal evolution extent
Summary
Oil and gas fields with recoverable reserves of more than 500 9 106 bbl of oil equivalent are referred to as giant oil and gas field (Halbouty 2003; Bai 2006). Marine carbonate rocks have considerable oil and gas potential (Jia et al 2006; Gu et al 2012; Bai and Xu 2014). Most of the discovered oil and gas reserves in carbonate rocks are mainly distributed in the. 2.3% of the total carbonate oil and gas reserves in the oldest strata have been found, because exploration has been very limited there and great potential exists for exploration, in China and the Asia–Pacific hydrocarbon-rich regions (Yu et al 2012; Jin et al 2013; Bai and Cao 2014). A clearer understanding of the geologic conditions and distribution characteristics of these giant oil and gas fields would offer valuable insights for further exploration of oil and gas resources in the PLP marine carbonate strata worldwide
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