The origin and accumulation of multi-phase reservoirs in the east Tabei uplift, Tarim Basin, China

Highlights

• Late gas invasion into paleo oils led to the changes of hydrocarbon phases.

• The intensity of gas invasion controlled the variation of fluid features and phases.

• Liquid hydrocarbon can be preserved due to steady rapid subsidence.

Abstract

A giant deep-strata oil-gas field with complex fluid characteristics was recently discovered in the Hade-Yuke area of the east Tabei uplift (Tarim Basin, NW China). The deep fluids show a lateral co-existence of several gas, condensate and oil phase reservoirs characterized through the integration of complimentary geochemical (e.g., bulk composition, biomarker and stable carbon isotope) and geological data. Typical features of the gas and condensate reservoirs, which were mainly distributed in the eastern parts of the study site, included high GOR (gas-to-oil ratio), hydrocarbon gases with relatively heavy δ¹³C values and crude oils with high wax content. In contrast oil reservoirs, which mostly occurred in more westerly locations, had lower GOR values, lighter δ¹³C values and lower wax content, indicating gradual change of fluid characters. The petroleum accumulations derive from two hydrocarbon charge events: an early oil charge in the Late Hercynian and a later gas charge in the Late Himalayan which migrated from east to west, leading to a gradual variation in fluid phase and characteristics. Alteration of paleo oil reservoirs by this secondary gas charge was therefore primarily responsible for the complex fluid character and multi-phase reservoirs presently in place. Secondary condensates were formed from the retrograde condensation of paleo reservoirs. Our re-construction of the hydrocarbon accumulation process encourages future exploration endeavours to target commercially viable gas accumulations in the deep east strata and oil reservoirs in the west.

Introduction

The formation of multi-phase reservoirs in the subsurface is affected by temperature/pressure conditions and in particular, the geochemical compositions of hydrocarbon fluids (Danesh, 1998; Zhou, 2004; Zhang et al., 2011a), which are greatly impacted by source rock organofacies, thermal maturation and multiple secondary alterations. With increased maturity the type of petroleum will change, for instance, oil phase exists in the mature stage (Ro ranges from 0.5% to 1.2%) while condensate and wet gas are generated subsequently after Ro exceeds 1.2%. Thermal maturation evolution is also significant for the change of reservoir phases through a set of thermal simulation of source rocks (Zhou et al., 1998). However, studies of many petroliferous basins worldwide demonstrated that after their formation, reservoir phases could change and differentiate under various physical-chemical impacts and/or by the changes in temperature-pressure conditions through tectonic activities. The main secondary alterations include migration fractionation, evaporate fractionation, thermal cracking and gas invasions. Migration fractionation was initially proposed by Gussow (1954) and Silverman (1965), demonstrated by later studies of Offshore Taiwan (Dzou and Hughes, 1993), Vermilion Block 39 field (Curiale and Bromley, 1996), Lunnan field in Tarim Basin (Zhang et al., 2011b), and North Sea (Larter and Di Primio, 2005). In this process, fractionation of light and heavy compositions occurs in the oil-gas migration process, subsequently leading to accumulations of divergent phases types. Evaporate fractionation was proposed by Thompson (1983, 1987, 1988), based on the different solubility of oil compositions in natural gas (Zhuze et al., 1962), and has been confirmed by studies of North Sea oilfield, Norway (Knudsen and Meisingset, 1991), the Norwegian continental shelf (Van Graas et al., 2000), the Suez Bay (Khavari-Khorasani et al., 1998), the eastern Carpathians (Matyasik et al., 2000), the Barrandian Basin (Matyasik et al., 2000), and the Northwestern Java Basin (Napitupulu et al., 2000). The excessive charge of dry gas during evaporate fractionation will lead to the loss of light and even middle compositions of oils by evaporate into gas phase. Besides, other concepts have been developed to describe the changes of hydrocarbon phases, for example, Behar et al. (1992), Horsfield et al. (1992), Berner et al. (1995) and Hill et al. (2003) have conducted simulations for kinetic process of crude oil cracking in confined systems and proposed that the generation of light hydrocarbons, condensates, dry gases and pyrobitumen through oil cracking in high-temperature conditions can lead to the changes of hydrocarbon phases. The concentration of diamondoids (Chen et al., 1996; Dahl et al., 1999; Wei et al., 2007; Zhang et al., 2011b; Zhu et al., 2014) and a series of methylbenzene compounds (Fusetti et al., 2010) are effective indicators of oil cracking. Meulbroek et al. (1998) put forward the concept of gas washing to explain the fractionation of oil composition and the changes of hydrocarbon phases during the late gas charging process in the South Eugene Island Block 330. Based on the exponential relationship between molecular concentration of n-alkanes and the related carbon numbers observed in the crude oil samples (Kissin, 1987), Losh et al. (2002) further proposed the calculation of “the loss of n-alkanes” to evaluate the extent of gas washing. Studies in the South China Sea (Zhang and Zhang, 1991), the Tabei uplift (Zhang et al., 2011b; Zhu et al., 2013) and Tazhong uplift (Zhu et al., 2014) in the Tarim Basin also supported that gas invasion is one of the significant factors for the changes of hydrocarbon phases and the formation of condensates. Zhu et al. (2014) established evaluation methods for the gas invasion extent with geochemistry parameters of the Ordovician oils in Tazhong area, with which the reservoir types can be identified. In summary, secondary alterations can lead to changes of hydrocarbon phases, through which oil reservoirs will sequentially transition into condensates and even gas reservoirs, which further results in the formation of multi-phase reservoirs. Different kinds of secondary alterations may occur simultaneously in one area.

The Tarim Basin is the largest petroliferous basin in China with an area of 56 × 10⁴ km² (Li et al., 1996; Jia and Wei, 2002; Wang et al., 2013) and Ordovician formations are the most important exploration strata. The character and distribution of oil-gas is complex due to the deep burial, geologic ages and multi-cyclic superimpositions and alterations (Su et al., 2011; Zhao et al., 2012; Zhu et al., 2012). Recently an Ordovician carbonate oil-gas field with reservoir depth ranging from 6200 to 7200 m was discovered in the Hade-Yuke area in the east of Tabei uplift. The fluids are complex in oil-gas features and phases, presenting challenges to future hydrocarbon exploration, exploitation and resource prospecting. The comprehensive analysis of the origin of multiple reservoir phases and the prediction of fluids distribution is thus vital for the promotion of the hydrocarbon exploration and exploitation process, and the development of hydrocarbon accumulation theories.