Formation and distribution characteristics of Proterozoic–Lower Paleozoic marine giant oil and gas fields worldwide

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.

Impact of pore structure and fractal characteristics on the sealing capacity of Ordovician carbonate cap rock in the Tarim Basin, China

“…However, cap rock should not always be regarded as a totally impermeable unit that does not allow flow of hydrocarbons or other fluids (Li et al, 2005;Schmitt et al, 2013), because it may still contain relatively fine pore structures and narrow throats that allow some fluid circulation (Vavra et al, 1992;Rezaeyan et al, 2015). There are extensive studies of cap rocks composed of mudstone, shale and gypsum (Caillet, 1993;Armitage et al, 2010;Boulin et al, 2013;Corrado et al, 2014;Huq et al, 2017;Newell et al, 2017;Liu et al, 2017), but investigations on carbonate cap rocks are less abundant and have only started in recent years. Most of the few existing studies mainly focus on the identification and quantitative characterization of the carbonate cap rock (Lü et al, 2017;Wu et al, 2018a).…”

Basic characteristics and accumulation mechanism of Sinian−Cambrian giant highly mature and oil-cracking gas reservoirs in the Sichuan Basin, SW China

“…Such gas reservoirs are featured by huge buried depths: in the Gaoshiti–Moxi Area, the average buried depth of the Longwangmiao Formation gas field is over 4650 m, that of the Deng 4 Member is over 5000 m, and that of the Deng 2 Member is over 5400 m, all of which should be categorized as (super-)deep gas reservoirs (Figure 3). Such deep and ancient reservoirs are first found in China, and they are also uncommon even globally (Liu et al., 2017; Zou et al., 2014b).…”

The nature, type, and origin of diagenetic fluids and their control on the evolving porosity of the Lower Cambrian Xiaoerbulak Formation dolostone, northwestern Tarim Basin, China

“…Dolostone reservoirs are important components of carbonate hydrocarbon reservoirs in many petroliferous basins worldwide (Ehrenberg et al 2006;Li et al 2011a, b;Sonnenberg and Pramudito 2009;Sun 1995;Warren 2000;Zhao et al 2005), but dolomite genesis and the mechanism for generating dolostone reservoirs remain intensely debated (Hardie 1987; Kirmaci and Akdag 2005;Machel 2004;Morrow 1998;Warren 2000;You et al 2015). Recent geophysical research found the Lower Cambrian platform margin facies dolostones in the subsurface of the Tabei Uplift (Ni et al 2015), which made the Lower Cambrian Xiaoerbulak Formation dolostones become potential exploration targets for hydrocarbon reservoirs (Du and Pan 2016;Liu et al 2017). Limited by scarce well samples, researches on the Lower Cambrian dolostones were mainly conducted in the Sugetbulak outcrop area of the northwestern Tarim Basin, where abundant pores, bitumen, and plentiful pore-filling dolomites were found (Li et al 2011a(Li et al , b, 2015Song et al 2014).…”