Caifu Xiang

Differential Hydrocarbon Migration and Entrapment in the Karstified Carbonate Reservoir: A Case Study of the Well TZ83 Block of the Central Tarim Uplift Zone

Abstract The hydrocarbon migration and entrapment mechanism in the lower overlapped basins, occurring in the complex carbonate pore-fissure-fracture reservoirs, is one of the key problems that have to be solved for effective hydrocarbon exploration. The production, gas/oil ratio, and the composition of crude oils and natural gas in the TZ83 Well block are high at the intersection point of the NE and NW-strike faults and decrease gradually along the ridge of the structure. A basic model of the pore-fissure-fracture system is built according to the achievements in the research on carbonate karstification. Processes of hydrocarbon migration and entrapment in this system are analyzed, which indicate that an understanding of the complexness of differential hydrocarbon migration is the key to interpreting this phenomenon. The hydrocarbon must charge the nearest compartment before migrating further away to charge other compartments in its pathway in the complex pore-fissure-fracture system. As a result, the following two phenomena appear: (1) Gas is enriched near the hydrocarbon injection point and drives away the oil, which is enriched in the compartments farther from the injection point. (2) The complex gas–oil–water relationship is controlled by the lateral connecting networks. Based on this, this article shows that the fault intersection point is the injection point of the oil and gas, and the main pathway system is distributed along the ridge of the structure. The theory of deferential hydrocarbon migration in the pore-fissure-fracture system can be presented from two aspects: (1) In hydrocarbon exploration, the structure of the fissure-fracture system should be described first, and then the special distribution of gas–oil–water can be predicted according to the main charging point and the main pathway system. (2) Exploration should be confined to the hydrocarbon charging point and the main pathway systems. An explored area should not be abandoned merely due to failures in some wells.

Transient fluid flow in the Binbei district of the Songliao Basin, China: Evidence from apatite fission track thermochronology

The Songliao Basin is famous for the Daqing Oilfield, the biggest in China. However, no economic hydrocarbon reservoir has been found in the northeastern Binbei district. Its thermal history, which is of great importance for hydrocarbon generation and migration, is studied with apatite fission track (AFT) thermochronology. Samples with depositional ages of the late Cretaceous (∼108–73 Ma) are analyzed. The AFT ages of the samples from reservoir rock (depositional age > 76.1 Ma) fall between the late Cretaceous (72±5 Ma) and the early Eocene (41±3 Ma) period, indicating their total annealing after deposition. In contrast, two samples from the main seals of the Qingshankou (depositional age > 89.3 Ma) and the Nenjiang Formation (depositional age > 73.0 Ma) are not annealed or partially annealed (AFT ages of 97±9 Ma and 70±4 Ma, respectively). Because the maximum burial temperature (<90 °C) evidenced by low vitrinite reflectance (Ro<0.7) is not high enough to account for the AFT total annealing (110–120 °C), the transient thermal effect arising from the syntectonic fluid flow between the late Cretaceous and the early Eocene is proposed. Transient thermal effects from fluid flow explains the indicated temperature discrepancies between the AFT thermometer and the Ro thermometer because the transient thermal effect from the fluid flow with a temperature high enough (110–120 °C) to anneal the AFT thermometer does not last long enough (104–105 yrs.) for an enhancement of the Ro (minimum 106–107 yrs. under the same temperature). This indicates that dating thermal effect from fluid flow might be a new means to reconstruct the tectonic history. It also answers why the samples from the main seals are not annealed because the seals will prohibit fluid flow and supply good thermal insulation. The large-scale fluid flow in the Binbei district calls for a new idea to direct the hydrocarbon exploration.

Factors controlling hydrocarbon reservoir formation and distribution and prediction of favourable areas in the Jurassic Toutunhe Formation in the Fudong slope, Junggar Basin, China

In recent years, many oil and gas reservoirs have been discovered in the Jurassic Toutunhe Formation in the Fudong slope, Junggar Basin. The total reserve in the formation is nearly 100 million tonnes, indicating huge exploration potential in the area. However, the main factors controlling the hydrocarbon accumulation remain unclear, and the mechanisms of hydrocarbon accumulation and enrichment are controversial. In this work, by studying the mechanism and main factors controlling hydrocarbon accumulation in the Toutunhe Formation in the Fudong area, a quantitative model of the reservoir rock and caprocks that control hydrocarbon distribution was constructed, the critical conditions for hydrocarbon accumulation were determined and favourable exploration areas were predicted. The results indicated that the oil in the Toutunhe Formation was derived from both Permian and Jurassic source rocks, the oil migrated from west to east after generation and both reservoir rock and caprock controlled the formation and distribution of oil reservoirs. The oil-bearing capacity improved with increases in the storage coefficient and caprock thickness. Accumulation did not occur when the reservoir rock and caprock index (RCI) was ≤ 0.5 but was favoured when the values were higher; this factor was the critical condition for hydrocarbon accumulation.