Qiu Nansheng

Tectono-thermal evolution of the Qaidam Basin, China: evidence from Ro and apatite fission track data

In the Qaidam Basin, on the Northeastern Qinghai–Tibet Plateau, the thermal evolution of western and northern sub-basins was analysed using apatite fission track (AFT) data and vitrinite reflectance data from 27 wells. This study shows that thermal gradients decreased during the evolution of the basin, but there existed some differences in thermal evolution between the west and north of the basin, due to different tectonic movements. The thermal gradients in Paleocene–Miocene in the west and in Jurassic times in the north were more than 30°C km−1, exceeding present-day gradients. These thermal data can provide useful parameters for the study of the Qaidam Basin and Qinghai–Tibet Plateau. Palaeotemperature data are the critical parameter for hydrocarbon generation calculations. The results in this study provide a foundation for hydrocarbon generation history modelling and petroleum resources assessment in the Qaidam Basin.

Tectono-thermal evolution of the Junggar Basin, NW China: constraints from Ro and apatite fission track modelling

The thermal evolution of the Junggar Basin, northwest China, was evaluated based on the thermal modelling results of 59 wells by using vitrinite reflectance (Ro) and apatite fission track (AFT) data. The thermal history indicates a cooling process of the basin since the Permian, but some differences in thermal evolution existed among the six structural units of the basin due to tectonic movements. The Junggar Basin was a ‘hot basin’ during the Permian, after which a cooling process with normal heat flow values occurred during the Mesozoic. Then the basin became a ‘cool basin’ from the beginning of the Tertiary. The average heat flow of the whole basin was 80 mW m−2 at the beginning of the Permian, then it decreased to 68 mW m−2 at the end of the Permian, to 63 mW m−2 at the end of the Triassic, 55 mW m−2 at the end of the Jurassic, 50 mW m−2 at the end of the Cretaceous and 42 mW m−2 at the present day. The heat flow distribution of the basin at different geological times also shows the thermal evolution characteristics of the Junggar Basin. At the beginning of the Permian, the highest heat flow, 85 mW m−2, occurred in the central basin and the eastern part of the basin, but the lowest heat flow was distributed along the southern and western basin margins, down to 70 mW m−2. The heat flow values were between 45 mW m−2 and 65 mW m−2 at the end of the Jurassic, with the lower value of 45 mW m−2 at the southern basin margin. The highest heat flow value again occurred at the southern end of the Luliang Uplift, at the northern part of the Central Depression and at the Eastern Uplift area during that period. At the end of the Cretaceous, it was down to 40–55 mW m−2. The lowest heat flow occurred at the Southern Margin and in the Wulungu Depression, and the highest value in the Eastern Uplift area. The tectonic subsidence also supports this thermal evolution of the basin. The rapid decrease of heat flow during the Tertiary in the Southern Margin of the basin may be caused by the uplift of the Tianshan Mountain. These heat flow data can provide useful parameters for the study of the Junggar Basin. Palaeoheat flow data are the critical parameter for hydrocarbon generation calculations. The results of this study provide a foundation for hydrocarbon generation history modelling and petroleum resource assessment in the Junggar Basin, which are important factors in the exploration of the Wulungu Depression and the study of stratigraphic and subtle traps in the Central Depression.

Phase states of hydrocarbons in Chinese marine carbonate strata and controlling factors for their formation

Chinese marine strata were mainly deposited before the Mesozoic. In the Tarim, Sichuan and Ordos Basins, the marine source rocks are made of sapropelic dark shale, and calcareous shale, and they contain type II kerogen. Because of different burial and geothermal histories, the three basins exhibit different hydrocarbon generation histories and preservation status. In the Tarim Basin, both oil and gas exist, but the Sichuan and Ordos Basins host mainly gas. The Tarim Basin experienced a high heat flow history in the Early Paleozoic. For instance, heat flow in the Late Cambrian varied between 65–75 mW/m2, but it declined thereafter and averages 43.5mW/m2 in the current time. Thus, the basin is a “warm to cold basin”. The Sichuan Basin experienced an increasing heat flow through the Early Paleozoic to Early Permian, and peaked in the latest Early Permian with heat flows of 71–77 mW/m2. Then, the heat flow declined stepwise to the current value of 53.2 mW/m2. Thus, it is a generally a high heat flow “warm basin”. The Ordos Basin has a low heat flow for most of its history (45–55 mW/m2), but experienced a heating event in the Cretaceous, with the heat flow rising to 70–80 mW/m2. Thus, this basin is a “cold to warm basin”. The Tarim Basin experienced three events of hydrocarbon accumulations. Oil accumulation formed in the late stage of Caledonian Orogeny. The generation and accumulation of oil continued in the Northern and Central Tarim (Tabei and Tazhong) till the late Hercynian Orogeny, during which, the accumulated oil cracked into gas in the Hetianhe area and Eastern Tarim (Tadong). In the Himalaya Orogeny, oil cracking occurred in the entire basin, part of the oil in the Tabei and Tazhong areas and most of the oil in the Hetianhe and Tadong areas are converted into gas. In the Sichuan Basin, another triple-episode generation and accumulation history is exhibited. In the Indosinian Orogeny, oil accumulation formed, but in the Yanshanian Orogeny, part of the oil in the eastern Sichuan Basin and most of the oil in the northeastern part was cracked into gas. In the Himalayan Orogeny, oil in the entire basin was converted into gas. The Ordos Basin experienced a double-episode generation and accumulation history, oil accumulation happened in the early Yanshanian stage, and cracked in the late stage. In general, multiple phases of heat flow history and tectonic reworking caused multiple episodes of hydrocarbon generation, oil to gas cracking, and accumulation and reworking. The phases and compositions of oil and gas are mainly controlled by thermal and burial histories, and hardly influenced by kerogen types and source rock types.

Thermal history of the Sichuan Basin, SW China:Evidence from deep boreholes

The Sichuan Basin is a superimposition basin composed of terrestrial and marine sediments that is well known for its abundant petroleum resources. Thermal history reconstruction using paleogeothermal indicators, including vitrinite reflectance and thermochronological data, shows that different structural subsections of the Sichuan Basin have experienced various paleogeothermal episodes since the Paleozoic. The lower structural subsection comprising the Lower Paleozoic to Middle Permian (Pz-P2 successions experienced a high paleogeothermal gradient (23.0–42.6°C/km) at the end of the Middle Permian (P2, whereas the upper structural subsection comprising Late Permian to Mesozoic strata underwent a relatively lower paleogeothermal gradient (13.2–26.9°C/km) at the beginning of the denudation (Late Cretaceous or Paleocene in the different regions). During the denudation period, the Sichuan Basin experienced a successive cooling episode. The high paleogeothermal gradient resulted from an intensive thermal event correlated to the Emeishan mantle plume. The heat flow value reached 124.0 mW/m2 in the southwestern basin near the center of the Emeishan large igneous province. The low geothermal gradient episode with heat flow ranging from 31.2 to 70.0 mW/m2 may be related to the foreland basin evolution. The cooling event is a result of the continuous uplift and denudation of the basin.