Development of Diapirs and Accumulation of Natural Gases in Yinggehai Basin

Overpressure has been found in about 180 sedimentary basins (Law and Spencer, 1998; Hunt, 1990). The mechanisms for the overpressured generation and the fluid flow activities in overpressured basins have been the subjects of a number of studies (Hunt, 1996; Ortoleva, 1994). Probably, the most important aspect of fluid flow activities in overpressured sedimentary basins is episodic fracture of sediments and cross-formation migration (Holm, 1998). Here, we present a case study to show that overpressure is caused mainly by disequilibrium compaction in a still-subsiding basin forced by formation of fractures and deformation of overlying intervals, resulting in both conduits and traps for fluid migration and accumulation with episodic injections of fluids through the conduits into the traps.

GEOLOGICAL SETTING

The Yinggehai and Qiongdongnan basins, separated by No.1 fault, are important Tertiary basins in the northern continental shelf of the South China Sea. The Qiongdongnan basin, eastern to northeastern trend, is a multi-stage rifting basin. However, the Yinggehai basin, northwestern trending, is a transform-extensional basin whose development was controlled by the combination of the lithosphere extension and strike-slip movement along the Red River fault zone (Gong and Li, 1997).

The Yinggehai basin is characterized by high subsidence rates (500-1 400 m/Ma), and the maximum thickness of Tertiary-Quaternary is over 17 km. The Neogene-Quaternary is dominated by fine-grained sediments, and no fault except small-faults associated with diapirs has developed. The thermal gradient of the Yinggehai basin is about 46 ℃/km, obviously higher than the average thermal gradients (30 ℃/km) of sedimentary basins of all ages in the world.

DISTRIBUTION AND CAUSE OF OVERPRESSURE

Overpressure developed over most of the basin. The measured pressure varied with depth (Fig. 1). Pressure calculation from seismic data shows that the burial depth to the top of overpressure varies from 1 600 m in the depocenter to more than 4 500 m on the basin margin.

Figure  1.  Pressure profile for Yinggehai basin.

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Several mechanisms have been proposed for the generation of overpressure in sedimentary basins, such as disequilibrium compaction, tectonic stress, hydrocarbon generation and aquathermal expansion (Hao et al., 1998). Although the effectiveness of each specific mechanism for overpressure generation is still in dispute, the most accepted mechanism of overpressure generation seems to be disequilibrium compaction and hydrocarbon generation (Hunt, 1996). The overpressured sediments display high interval transit time (Δt) (Fig. 1) and, therefore, have high porosity, suggesting that the overpressure in the Yinggehai basin is mainly caused by disequilibrium compaction (Swarbrick and Osborne, 1998). In addition, the overpressured sediments usually have low organic matter contents (TOC < 0.6) with a wide variety of maturity levels from immature to post-mature, suggesting that hydrocarbon generation could not have been the main mechanism for overpressure development in the basin.

A closed or semi-closed fluid system in the overpressured interval may have helped to stabilize the high overpressure. As shown in Fig. 2, the overpressured intervals in well LD3011 show high production indices (PI=S₁/ (S₁+S₂)) (0.3 to 0.8), indicating that the deep, strongly overpressured system in well LD3011 is a closed system, and that most of the generated hydrocarbons have been still remained in the source rocks or nearby siltstones. The closed or semi-closed fluid system could also be inferred from the facts that the overpressured interval shows high gas test values (Fig. 2) and that sandstones in the overpressured interval at about 5 000 m still have high porosities. The development of the closed or semi-closed system was a combined result of presence of fine-grained sediment-fill and absence of major faults (Hao et al., 1998; Price, 1994).

Figure  2.  Organic geochemistry profile for well LD3011. Note the high S₁/ (S₁+S₂) values and abnormally low t_maxvalues in overpressured intervals.

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In summary, the overpressured generation in the Yinggehai basin is mainly caused by disequilibrium compaction due to high-rate sedimentation of fine-grained sediments. Closed or semi-closed fluid system developed due to lack of faults in fine-grained sediments helped to stabilize the strong overpressure.

DEVELOPMENT OF DIAPIRS AND FORMATION OF FLUID CONDUITS AND TRAPS

The central depression zone of the basin with the highest subsidence-sedimentation rates, and the finest sediments, is of the strongest overpressure and the smallest burial depth (1 600-2 000 m) to the top of overpressured systems. Off the subsidence center, the burial depth of the top of overpressured system increased (3 600-4 300 m). Therefore, the top boundary of the overpressured system in the entire basin was not flat, but fluctuating. Such overpressure distribution patterns are significant to the control of the rock and fluid activities.A stratum will be disrupted and fluids will be released when fluid pressure reaches the fracture pressure (corresponding to the minimum horizontal stress, usually believed to be about 85 % of the lithostatic pressure) (Roberts and Nunn, 1995). Although more and more researchers consider the cyclic fracturing of the top boundary of the overpressured systems as the main mechanism of overpressured fluid release, a few scholars have discussed the fracture points of the overpressured systems. The fluctuation of the top boundary of the overpressured systems indicates that the fluid pressure at the same burial depth (where the lithostatic pressure is similar) is not equal throughout the basin, which determines the location of hydrofracture. As shown in Fig. 3a, the uplift point (point A in Fig. 3a) of the top surface of the overpressure shows the smallest difference between fracture pressure and formation pressure. As the burial depth of the top surface of the overpressure increases (point B for example), the difference between fracture pressure and formation pressure increases. As a result, the formation pressure at the uplift point of the top surface of the overpressure more easily reaches the fracture pressure, leading to fracturing and fluid expulsion. The right-lateral slip of the boundary fault of the Yinggehai basin resulted in a near north-south regional extension stress field (Fig. 3b). When the fluid pressure reached the fracture pressure, the formation fractured along the main extensional stress axis. Fluids and some plastic shales released through the fractures, resulting in the formation of the diapirs. Therefore, the diapirism in the Yinggehai basin was a combined result of strong overpressure (caused mainly by rapid subsidence of the basin and sedimentation of fine-grained sediments) and the regional tensile stress field (determined by the long-term right-lateral slip of the boundary fault), the subsidence-sedimentation rates determined the uplift point of the top surface of the overpressure, and, therefore, determined the location of the diapirs, whereas the regional tensile stress controlled by the right-lateral slip of the boundary fault determined the distribution and arrangement of the diapirs. As a result, diapirs with 5 rows in en echelon arrangement (Fig. 3b), developed at the subsidence centers of each period associated with the highest subsidence-sedimentation rates, the finest sediments and the strong uplift of the top surface of the overpressure.

Figure  3.  (a) Schematic diagram showing illustrating control of uplift of overpressure top on hydrofracture and fluid release point; (b) Distribution of diapirs in Yinggehai basin and its relation to subsidence centers. SF.sea floor; QDN. Qiongdongnan.

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The diapirism in the Yinggehai basin is of multiple-stage and cyclic nature. In the later episode of diapirism is captured the hydrofractures that formed during the earlier episode of diapirism, and the multi-episode diapirism led to the formation of irregularly and compactly distributed faults with small fault throws (Fig. 4). These faults comprised the main conduits for rapidly vertical migration of fluids including oil and gas.

Figure  4.  Pre-stack depth migration seismic section (97-3642) showing small-throw faults associated with diapirs.

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Diapirism also led to the deformation of the overlying intervals (Fig. 4), resulting in the traps at the depocenter which could not have existed without overpressure-induced diapirism.

EPISODIC NATURAL GAS ACCUMULATIONS

A number of gas accumulations have been found in the dia-pir structure zones in the Yinggehai basin. The diapir would act as an "outlet" for the release of fluids from overpressured systems, and the traps over the diapir might act as "containers" receiving fluids episodically released through the diapir. Geological, geothermal and geochemical data from the gas field over the diapir might provide evidence for the episodic injections of fluids.

The gas fields in the diapir structure zones display considerable inter-reservoir compositional heterogeneities (Fig. 5). The sharp changes in gas compositions in the same gas field suggest that the injections of fluids into the reservoirs must have been non-continuous, which could be interpreted as intermittent, or "episodic".

Figure  5.  Cross section of Dongfang gas field showing inter-reservoir compositional heterogeneities and variation of clay mineral transformation profiles.

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The transformations of clay minerals in sandstones are strongly enhanced (Fig. 6) by hydrothermal fluid flow. However, the hydrothermal fluid flow has not affected the clay diagenesis in the shales overlying or underlying the reservoir sandstones (Fig. 6), showing that the thermal effects of the fluid injections were shortly-lived, transient in nature, characteristic of intermittent fluid flow.

Figure  6.  Anomaly of clay mineral diagenesis in sandstones caused by overpressured fluid activities in well DF113 in the Yinggehai basin.

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The wide variation of w (toluene)/w (n-heptane) values also suggests episodic fluid injections. The C₇ hydrocarbon distribution is a sensitive parameter reflecting migration fractionation processes (Curiale and Bromley, 1996; Whelan et al., 1994; Thompson, 1988, 1987). As shown in Fig. 7, all samples from reservoirs with inorganic CO₂ and high CO₂ contents (> 50 %) havew (toluene)/w (n-heptane) values significantly higher than those from reservoirs with organic CO₂ and low CO₂ contents (< 10 %), indicating that hydrocarbons from reservoirs with high CO₂ contents have undergone higher degrees of migration fractionation. This is consistent with the reservoir-filling processes that CO₂-rich fluids invaded the reservoirs after the accumulation of methane-dominated gases. With the injection of CO₂-rich gases, the accumulated methane-dominated gases migrating from the reservoirs were gradually displaced, resulting in the removal of saturated hydrocarbons with low fugacity, leading to hydrocarbons accumulation in the CO₂-rich gases are rich in aromatic hydrocarbons (high w (toluene)/w (n-heptane) values). Compared with the experimental results of Thompson (1987), the high w (toluene)/w (n-heptane) values reflect multi-stage fractionation, and can be readily attributed to the results of episodic fluid injections, with each episode of fluid injection followed by migration fractionation.

Figure  7.  Variation of w (toluene)/w (n-heptane) with w (n-heptane)/w (methylcyclohexane) for condensates from diapir structure zones in Yinggehai basin. Samples labeled by numbers are experiment results of Thompson (1987), the number expresses experiment stage.

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CONCLUSSIONS

The strong overpressure in the Yinggehai basin is caused mainly by disequilibrium compaction due to rapid sedimentation of fine-grained sediments. Closed or semi-closed fluid system development due to the absence of major faults in fine-grained sediments also helps to stabilize the overpressure. The overpressure in the Yinggehai basin is a dynamic system with non-flat top surface. As a combined result of strong overpressure and tensile stress field caused by strike-slip movement of the boundary fault, diapirs developed at the depocenter where the sedimentation rates were the highest with the shallowest overpressure top. The diapirism led to the formation of vertical conduits and traps at the depocenter. The accumulation of natural gases in the reservoirs over the diapirs was intermittent or episodic, which could be supported by the inter-reservoir compositional heterogeneities and the transiently thermal effects of fluid injections.