Dynamic continuous hydrocarbon accumulation (DCHA): Existing theories and a new unified accumulation model

Abstract

Replacing coal with cleaner hydrocarbon resources is a viable solution to reach carbon neutrality goals over time, especially given the current lack of growth in green energy resources (e.g., hydro, solar, wind, geothermal) globally. Hydrocarbon resources offer a low-carbon, low-waste, large-scale, rapid solution. Dynamic continuous hydrocarbon accumulation (DCHA) exists widely in sandstone and shale reservoirs and has enormous resource potential. However, understanding the accumulation mechanisms and models of DCHAs in different structural locations is highly challenging. In this study, we first extensively review hydrocarbon resources derived from hydrocarbon accumulation and identify key scientific issues and theories. We then take the well-known typical DCHA in the Bohai Bay Basin in eastern China as a case, and combine multiple analytical methods, such as biomarker analysis, petroleum and aqueous inclusion analysis, quantitative fluorescence analysis, microscopic thin section observations, and basin modeling, to investigate petroleum sources, charging times, and charging forces. The results of our study show that the DCHA is a superimposed accumulation of multiple petroleum reservoir types with multiple petroleum sources, charging times, charging forces, and modes. Petroleum reservoirs at high structural locations are conventional trap reservoirs accumulated by buoyancy in the early stage, with long migration distances. Petroleum reservoirs at the sag location are deep unconventional petroleum reservoirs accumulated by the petroleum generation pressurization force in the middle stage, with short migration distances. Petroleum reservoirs at the slope location are superpositions of conventional traps and deep unconventional petroleum reservoirs in the middle and late stages, with moderate migration distances.

Introduction

Petroleum is the most important primary energy source on Earth. Its proportion in the global primary energy supply is expected to remain at >50% by 2040 (EIA, 2019; Exxon, 2018). The world's carbon emissions are set to peak in the next few decades. To achieve China's carbon peak and carbon neutrality goals by 2030 and 2060, respectively (Liu, 2015b; Zhang et al., 2021), replacing coal with hydrocarbons as soon as possible to replenish weak growth in clean energy is one of the most viable approaches. However, this approach requires further strengthening of petroleum exploration and development.

Since the world's first industrial oil well was successfully drilled in 1859 (Jia, 2017), the global petroleum industry has experienced >160 years of development. During this period, corresponding to three significant petroleum exploration discoveries, three important petroleum geological theories were established. White (1885) proposed the “Anticlinal Theory for Petroleum Accumulation” which stated that buoyancy is the driving force for petroleum accumulation, and oil, gas and water are differentiated according to their densities. Low-density oil and gas occupy the top of the anticline, while high-density water is at the bottom. Therefore, the top of the anticline is considered to be the best area for petroleum exploration. Guided by this theory, a large number of anticlinal hydrocarbon resources have been discovered. Mccollough (1934) discovered that petroleum accumulation requires traps, including reservoirs, caprocks and blocking conditions, which have uniform oil, gas and water interfaces and proposed the “Trap Theory for Petroleum Accumulation”. Guided by this theory, a large number of lithologic, stratigraphic and fault petroleum reservoirs located in structural slopes have been discovered. More recently, “Continuous Petroleum Accumulation Theory” (Schmoker, 1995) and “Unconventional Petroleum Theory” (Law and Curtis, 2002a) have promoted the large-scale discovery of tight and shale petroleum reservoirs, respectively.

Dynamic continuous hydrocarbon accumulation (DCHA) refers to extensive petroleum accumulation in both sandstone and shale reservoirs, which is a dynamic process that exists continuously in different structural locations (highs, slopes, and sags) within a single layer or multiple layers in a petroliferous basin (Schmoker, 1995; Li et al., 2017; Pang et al., 2014b). In addition to the petroleum accumulated in unconventional tight reservoirs, DCHAs include genetically related petroleum accumulated in conventional reservoirs. DCHAs exist in large areas with no obvious oil-gas-water interface boundaries (Zou et al., 2013, Zou et al., 2019), and have unusual mechanisms for petroleum accumulation and interstitial flow (Etherington and McDonald, 2004; Guo et al., 2017; Law and Curtis, 2002b; Rose, 1981; Spencer and Mast, 1986).

Global petroleum exploration practices have shown that DCHAs widely exist in deep strata adjacent to source rocks in petroliferous basins and have great resource potential (Brown et al., 1982; Zhang et al., 2015; Zhao, 2012; Zou et al., 2013, Zou et al., 2015; Craig et al., 2018; Cao et al., 2020; Li et al., 2021) (Fig. 1). Studies on the genetic mechanisms and distribution of individual DCHA reservoirs (Jin and Zhang, 1999; Law and Curtis, 2002a; Zhang, 2003; Zhang et al., 2003) have achieved considerable progress. They include conventional anticline (high structural locations), lithological-stratigraphic and fault (structural slopes and high locations), and tight (structural slopes and sag locations) petroleum reservoirs (Ayers Jr, 2002; Law and Curtis, 2002b; Montgomery et al., 2002; Schmoker, 2002). However, these separate studies on individual DCHA reservoirs have failed to systematically uncover intrinsic and substantial relationships among different types of petroleum reservoirs in different structural locations, resulting in a lack of systematic and coherent understandings of some key issues in petroleum geology theory. For example, the petroleum properties in DCHAs in different structural locations show clear progressive changes. Specifically, as the structural locations change from high to slope to sag, 1) the petroleum phases change from heavy oil to light oil, to condensate oil, and finally to dry gas, 2) the oil-gas-water interfaces change from a normal interface (gas at the top, oil at the middle, and water at the bottom) to an inverted oil-water interface (water at the top and oil at the bottom) to an inverted oil-gas interface (oil at the top and gas at the bottom) and finally to no gas-water interface (gas and water completely mixed), 3) the density and viscosity of oil decrease gradually, and 4), the gas-oil ratio (GOR) and thermal maturity increase gradually. This phenomenon is common in the Bohai Bay Basin (Cheng et al., 2015; Hu, 2019; Jin and Zhang, 1999; Zhang, 2003), Songliao Basin (Wu et al., 2007), Sichuan Basin (Wu et al., 2007), Ordos Basin (Zhao, 2012; Zhao et al., 2013; Zhao et al., 2016; Zou et al., 2013), Tuha Basin (Zhang et al., 2003), Junggar Basin (Hu et al., 2018; Zhi et al., 2021), and Tarim Basin (Pang et al., 2014b) in China, the Chu-Salesu Basin (Pang et al., 2014b) in Kazakhstan, and the Fort Worth Basin (Pollastro, 2007) and San Joaquin Valley (Larue et al., 2018) in the USA. Studies on the accumulation process for different types of petroleum reservoirs in DCHAs in different structural locations are derived only from hypotheses about geological phenomena and are subjective. Specifically, understandings of DCHAs formed by superimposition with different types of petroleum reservoirs accumulated in different periods are mainly based on the characteristics of basin tectonic evolution and source rock-reservoir assemblages, but little effort has been made to clarify the accumulation history of DCHAs (Guo, 2014; Jiang et al., 2006; Pang et al., 2014b; Pang et al., 2021; Pang et al., 2014b; Pang et al., 2000; Pang et al., 2021a; Schenk, 2005; Tao et al., 2011; Yang et al., 2022; Zhang et al., 2003).

In the Dongpu Depression, Bohai Bay Basin, China, Paleogene petroleum exploration from anticline, lithological, stratigraphic, and fault hydrocarbon reservoirs in high structural locations has entered a highly mature stage (Duan et al., 2008), and the remaining exploration has gradually advanced to the structural slope and sag locations. Commercial oil and gas flows have been obtained from many wells targeting Paleogene tight sandstone reservoirs on the slope and sag. Furthermore, statistical oil test data of exploratory wells near the sag show that almost all wells drilled in the tight sandstone reservoirs have noticeable oil and gas shows although oil and gas yields are generally low, and commercial wells are limited. This fact indicates that the abundance of oil and gas in the slope and sag is low, but the distribution area is large. As the structural location deepens from high to slope to sag, the petroleum phase, oil-gas-water interface, density, viscosity, and GOR show clear continuous changes. The petroleum accumulated in the Paleogene Shahejie Formation of the Dongpu Depression is distributed from deep to shallow and from bottom to top and shows the basic geological characteristics of a DCHA. As a well explored area, the Dongpu Depression, especially the Shahejie Formation, together with its huge exploration data, provides a good opportunity for addressing issues related to the genetic mechanism and distribution patterns of DCHAs, including the intrinsic relationships among different types of petroleum reservoirs in DCHAs in different structural locations, progressive accumulation processes, and accumulation models.

In this study, we first reviewed DCHA theories, and highlighted key scientific issues that need to be solved. Then we considered a typical DCHA in the Dongpu Depression as a case study and used analytical data acquired from oil, shale and sandstone drill samples, such as biomarker analysis, petroleum and aqueous inclusion analysis (petrographic and homogenization temperature), quantitative fluorescence analysis, and microscopic thin section observations, to determine the oil sources, charging times, charging forces, and basin modeling for the DCHA at different structural locations. Finally, the above studies were combined with the structural and depositional settings to reconstruct the charging history and establish a new unified accumulation model for DCHA exploration.