Preservation and Long Life of Ancient Deep Oil Reservoirs under the Background of Low Geothermal Gradient and Late-Period Rapid Burial

Guangyou Zhu¹, Shuichang Zhang¹, Jin Su¹, Keyu Liu¹, Haijun Yang², and Kun He^1
1Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China
²Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Xinjiang Korla, China

Stability of crude oil is a hot topic and most of scholars believed that crude oil begin to crack when temperature is above 160°C. However, exploration for deep oil and gas shows that oil exists greatly in some oil pools far beyond this temperature. The stability of crude oil is possibly much higher than expectation. The large oil reservoirs of Halahatang and Rewapu are discovered in deep layers of 7000 m in the Tabei area of the Tarim basin in China. Geochemical research reveals that oils in these reservoirs are almost not cracked, it is expected that oil phase exists in even deeper laysers, and explorationists are concerned with the depth at which oil phase disappears.

Thermodynamic is the basis for the cracking of crude oil and plays a critical role in the cracking of crude oil. The temperature threshold of crude oil cracking into gas may also controlled by the following three factors besides temperature: (1) catalytic conditions in the fluids of the reservoirs (e.g. TSR), which can decrease temperature threshold for oil cracking and also can accelerate the cracking degree of crude oil; (2) nature and composition of crude oil; (3) compensation effect of time.

The Tarim basin is a “cool basin” now, and its present geothermal gradient is about 20°C/km. Gold tube simulation of the marine oil in the Tarim basin shows that, under the constant burial temperature of 180°C, crude oil needs 52.8Ma to convert 51%; but under general burial conditions (2°C/Ma), it will take 100Ma for crude oil to reach the same conversion rate. According to the study of compensation effect in the process of late period rapid burial and low geothermal gradient in the Tarim basin, large scale of oil cracking occurs when the burial depth is 9000~9500m. The great depletion of liquid oil (oil is cracked into gas) happens when reservoir temperature is higher than 210°C, and liquid oil may exist in great amount within 9000m. Obviously, compared with the continuous burial under normal conditions, the stable burial under high temperature is beneficial to oil cracking. The comparison between the Tarim basin and the Sichuan basin shows that the former reaches its maximum burial depth in recent 5Ma, while the latter reaches its maximum burial depth in the middle of the Cretaceous. So, the crude oil in the Sichuan basin has been highly cracked, which is closely related to long-term, high-temperature deep burial.

From the relationship of hydrocarbon generation, fluid inclusions and burial history, autogenic illite K-Aa dating, and other evidence in trap formation, it is determined that these oil reservoirs are primary, ancient oil reservoirs formed and preserved before 250Ma. The late Hercynian (Permian) is an important stage of hydrocarbon generation and expulsion in the Tarim basin and it is also the most effective period for hydrocarbon generation. Due to serious denudation of caprock caused by the tectonic activities before the Triassic deposition, oil reservoirs in structural highs are damaged and some oil is biodegraded, but oil and gas in periclinal areas are possibly preserved. Since the Triassic deposition, Ordovician oil reservoirs have been deeply buried, caprocks have continued to thicken, the location and shape of Ordovician traps have kept almost unchanged, oil and gas have been kept till present.

Therefore, the discovery of ancient oil reservoir in the Tarim basin has strengthened the confidence of finding primary marine oil reservoirs in complicated structural areas. Low geothermal gradient and rapid burial in late period are the major factors for the preservation of oil in deep layers. Recent marine exploration focuses on uplift slopes and platforms, the main exploration depth may reach 9000m, and the potential of exploration for deep oil increases.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013