Jiagui Shen

Determination of trapping pressure and temperature of petroleum inclusions using PVT simulation software - a case study of Lower Ordovician carbonates from the Lunnan Low Uplift, Tarim Basin

Abstract We report a case study carried out on Lower Ordovician reservoir carbonates of the Lunnan Low Uplift, Tarim Basin, to determine trapping pressures and temperatures of petroleum inclusions from the intersection of isochores of coeval petroleum and aqueous fluid inclusions using PVT SIM software. There are four types of petroleum inclusions from this sequence. Their trapping pressures and temperatures are: Type I 80–105 °C, 199–230 bar; Type II 135–145 °C, 235–250 bar; Type III 155–165 °C, 344–360 bar, and Type IV 220–252 °C, 345–390 bar, respectively, which are consistently higher than their homogenization temperature and saturation pressures by about 5–15 °C and 25–70 bar. To confirm the reliability of the PVT simulation approach, fluid inclusions were synthesized in calcite under controlled temperature and pressure conditions and studied using the same methods. The calculated trapping pressure and temperature of the synthetic petroleum inclusions closely match the actual pressure and temperature applied in the experiment. By reference to the burial and geothermal history of the Tarim host rocks, it was inferred that Types I, II and III petroleum inclusions represent fluids associated with three distinct phases of petroleum emplacement in the Lower Ordovician strata of this area, occurring during late Silurian to early Devonian, Cretaceous to early Tertiary, and late Tertiary, respectively. Type IV petroleum inclusions occur only in some fine calcite veins from few samples, and are believed to be related to an anomalous thermal event in this area. On the basis of the trapping pressures and trapping temperatures of the petroleum inclusions, three main phases of the petroleum-filling of the Lower Ordovician reservoirs can be delineated. Phase I and phase III petroleum events were related to petroleum migration and accumulation from the Lunnan Low Uplift or nearby area. The petroleum of the phase II petroleum event appears to have originated from Middle and Upper Ordovician source rocks in the northern slope area of the Manjiaer Depression with a long distance migration.

An investigation of water-gas interface migration of the upper Paleozoic gas pool of the Ordos Basin using reservoir fluid inclusion information

There is a particular characteristic in the formation of the Upper Paleozoic gas pool in the Ordos Basin that is its water-gas interface migrated regional during geological history. However, there has been lack of detailed research on this problem and also no similar report on other basins. In this paper, the formation time of the fluid inclusions formed in the water-gas transition zone of the gas pool was deduced using their trapping temperatures and combining of the burial with geothermal history of the basin. On the basis of this, the isochrone of water-gas interface migration for the gas pool was mapped. The result shows that the gas pool began to form around the Yanan Area at about 165Ma, and then developed and enlarged toward the north direction. The gas pool finally formed at about 129 Ma. Since the basin uplifted from the late Cretaceous and gas supply decreased, the water-gas interface of the gas pool migrated back to the present position.

The generation and accumulation of natural gas from Yinan 2 gas pool in Kuqa Depression

By using the methods of hydrocarbon generation kinetics and carbon isotope kinetics, combined with geological background of natural gas pool formation, the generation and accumulation of natural gas from Yinan 2 gas pool was studied in Kuqa Depression of the Tarim Basin. Natural gas of Yinan 2 gas pool is mainly derived from Middle and Lower Jurassic coal-bearing source rocks, and generally belongs to long time-accumulated gas. It is suggested that Yinan 2 gas is chiefly accumulated in the last 5 Ma, itsR0 ranges from 1.25% to 1.95%, and the loss rate of natural gas is about 25%–30%. This work not only complements and reduces the deficiency of formation model of natural gas pools which traditionally depends on the matching relationships between source rock, reservoir, cap rock, and trap, but also is a useful reference in the study of other gas pools.

Dating hydrocarbon generation and migration based on bitumen reflectance

On the basis of thermal maturation theories of organic matter in sediments and an improved Karweil’s method, a method for dating hydrocarbon generation and migration by means of bitumen reflectance has been suggested. A few representative boreholes in the Tazhong Area of the Tarim Basin was investigated by this method. The results indicate that the three phases of bitumen from the Tazhong Area formed during Middle and Late Ordovician, Late Cretaceous-Early Tertiary, and Late Tertiary, respectively. This implicates that there were three phases of hydrocarbon generation and migration occurring in this area during geological history. This study provided a new idea for the geological application of geochemical data of bitumen.

A new method to reconstruct hydrocarbon-generating histories of source rocks in a petroleum-bearing basin——the method of geological and geochemical sections

Via investigating typical Palaeozoic and Mesozoic petroleum-bearing basins in China by using thermal maturation theories of organic matter to improve the conventional Karweil’s method, a new method to reconstruct hydrocarbon-generating histories of source rocks has been suggested. This method, combining geological background with geochemical information makes the calculated VRo closer to the measured one. Moreover, it enables us to make clear the hydrocarbon generation trend of source rocks during geological history. The method has the merits of simple calculation and objective presentation, especially suitable to basins whose sedimentation and tectonic movements are complicated.

Observation of hydrocarbon generation and migration of highly-matured carbonates by means of laser-induced fluorescence microscopy

Some important information on hydrocarbon generation, inclusion and migration in highly-matured carbonates of lower Palaeozoic age from the Ordos Basin and Tarim Basin has been analyzed by a newly-combined laser-induced fluorescence microscope (LFM) designed by our laboratory. The following information has been obtained from the lower Ordovician lamellar carbonates with equivalent vitrinite reflectance (Ro) as high as 1.6%–1.7% and residual TOC of 0.14%–0.35% from the Ordos Basin: wide occurrences of oil and source macerais with strong fluorescence, includingG.Prisca alginite, lamalginite, telalginite and algae-detrinite; fluorescing asphalt among mineral crystals; some groundmass and spheroid-like reservoir bitumen with high maturation levels in the pores of dolomites. Various kinds of fluorescing organic inclusions and asphalt have been found in the carbonates, calcareous shales and silt-shales with high maturation levels from the Cambrian-Ordovician strata in the Tarim Basin. All this helps us find and evaluate significant and excellent source rocks for large- and middle-scale gas fields. The net and micro-net systems for hydrocarbon generation, expulsion and migration in carbonates have been revealed by the highly-powered laser-induced fluorescence microscopy.

A new method based on vitrinite reflectance gradient to determine paleotemperature gradient of a petroleum-bearing basin

Based on the Arrhenius equation and Karweil method, a simulation calculation has been made on the thermal maturation of organic matter under a series of assuming paleotemperature gradients. Results show that there was a positive correlation of vitrinite reflectance gradient with paleotemperature gradient and vitrinite reflectance. According to this, a model has been established which presents the quantitative relationship between three parameters. This model can be directly applied to determining the paleotemperature gradient of a petroleum-bearing basin with the measured vitrinite reflectance.

Characterization and origin types of immature and low-mature amorphous kerogen in terrestrial source rocks

The microfluorescent technique was applied to investigate the fluorescence alteration patterns of amorphous kerogen of the terrestrial source rocks of China. Two fluorescence parameters, I54620 andR were suggested, which can be taken to divide the amorphous kerogen into five types: algae amorphinite, biodegraded amorphinite A, B, C, and humic amorphous. On the basis of this, the Rock-Eval analysis results of source rocks were used to investigate the hydrocarbon generating potential of these amorphinite.

The generation kinetics of natural gases in the Kela-2 gas field from the Kuqa Depression, Tarim Basin, northwestern China

The Kela-2 gas field, found in the Kuqa Depression of the Tarim Basin, northwestern China, is a large-sized dry gas field (C1/C1–5=0.992–0.999) and characterized by ultra-high pressure (pressure factor up to 2.0–2.2). The pyrolysis experiment was carried out under isothermal gold-tube closed system, with samples collected from the Jurassic coal, Jurassic mudstone and Triassic mudstone in the Kuqa Depression. The result of gas yield showed that the Middle and Lower Jurassic source rocks have higher gas generation potential than the Triassic source rocks. The kinetic modeling of gas generation and methane carbon isotope fractionation suggested that the Kela-2 gases belong to the products of high-over mature stages and were mainly derived from the Middle and Lower Jurassic coal-bearing strata. The Triassic source rocks made a minor contribution to the Kela-2 gases. The Kela-2 gases chiefly generated from coal-bearing source rocks with Ro values from 1.3% to 2.5%, and thus primarily accumulated after 5 Ma.