Zufa Liu

Laser-induced fluorescence microscopy - application to possible high rank and carbonate source rocks

Abstract It is difficult to obtain useful fluorescence information from coals and source rocks of high maturity by means of conventional fluorescence microscopy due to the low fluorescence intensity of the macerals. A laser-induced fluorescence microscopy (LIFM) designed by the authors was applied to target the problem. An investigation was made by means of this technique on a suite of coals having a range of mean random vitrinite reflectance (VRo)=0.3–2.5%, and some high maturity source rocks of early Palaeozoic age. The results show that LIFM greatly improves several functions of conventional fluorescent microscopy. LIFM is very sensitive to organic matter in sediments and for the suite of Chinese coals and possible source rocks investigated, considerably extends the maturity range over which visible fluorescence can be observed. The range is extended from about 1.40% to 2.0% VRo for vitrinite of normal composition, from 1.30% to about 1.50% VRo for alginite and other liptinites, and from about 2.5% to 3.2% VRo for organic inclusions and asphalt-absorbed minerals. With LIFM, it was found that the organic matter in some early Palaeozoic carbonates and argillaceous limestones is distributed very unevenly. In these rocks, which are believed to have been significant source rocks, some microregions are very rich in organic matter or contain a large number of organic inclusions although the total organic carbon (TOC) of the whole rock is low. A microchannel network for petroleum expulsion and migration, comprised of grain boundaries, bedding planes and suture textures containing hydrocarbons was revealed in carbonate strata. LIFM of organic matter in rocks should extend the application of organic petrology in petroleum exploration.

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.