Carbonate bands are common in Central graben Kimmeridgian (Upper Jurassic) claystone that is mature for oil generation. X-ray diffraction, thermal analysis, Mossbauer spectroscopy, thin-section studies, scanning microscopy, and electron microprobe investigations showed that the bands are composed of claystone fragments, coarse grains of dolomite, quartz, and siderite, and diagenetic layer-silicates, all cemented to a certain degree by ankerite. The diagenetic layer-silicates are kaolinite being neoformed into muscovite. In claystones adjacent to the carbonate bands, ankerite fills pores about 50 µm in diameter; however, claystone further away from the bands contains little ankerite. Gas adsorption with N2, Kr, and H2O together with microscopy showed porosity in the claystone is composed of a large amount of micropores and mesopores, possibly with inlet constrictions, and a few macropores with diameters of about 50 µm. In the carbonate bands, microporosity and mesoporosity are probably located in the claystone fragments, whereas several pores with diameters of about 50 µm are present in the carbonate matrix. The presence of claystone fragments, fluid release structures, and large amounts of diagenetic carbonates indicates that carbonate bands formed through fracturing, possibly as a result of overpressuring. Porosity parameters indicate that fluid transport occurred by diffusion in the claystone matrix to claystone macropores and further through macropores in the carbonate bands, out of the source rock. The diagenesis of carbonates and clay minerals in the carbonate bands and in the claystone points to two migration stages: an early migration of fresh, neutral fluids, and a later migration in connection with hydrocarbon migration of saline-alkaline fluids.

Diagenesis and Primary Migration in Upper Jurassic Claystone Source Rocks in North Sea

The Tarim Basin in Northwest China hosts petroleum reservoirs of Cambrian, Ordovician, Carboniferous, Triassic, Jurassic, Cretaceous and Tertiary ages. The sedimentary thickness in the basin reaches about 15 km and with an area of 560 000 km 2 , the basin is expected to contain giant oil and gas fields. It is therefore important to determine the ages and depositional environments of the petroleum source rocks. For prospective evaluation and exploration of petroleum, palynological investigations were carried out on 38 crude oil samples collected from 22 petroleum reservoirs in the Tarim Basin and on additionally 56 potential source rock samples from the same basin. In total, 173 species of spores and pollen referred to 80 genera, and 27 species of algae and fungi referred to 16 genera were identified from the non-marine Mesozoic sources. By correlating the palynormorph assemblages in the crude oil samples with those in the potential source rocks, the Triassic and Jurassic petroleum source rocks were identified. Furthermore, the palynofloras in the petroleum provide evidence for interpretation of the depositional environments of the petroleum source rocks. The affinity of the miospores indicates that the petroleum source rocks were formed in swamps in brackish to lacustrine depositional environments under warm and humid climatic conditions. The palynomorphs in the crude oils provide further information about passage and route of petroleum migration, which is significant for interpreting petroleum migration mechanisms. Additionally, the thermal alternation index (TAI) based on miospores indicates that the Triassic and Jurassic deposits in the Tarim Basin are mature petroleum source rocks.

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Mesozoic non-marine petroleum source rocks determined by palynomorphs in the Tarim Basin, Xinjiang, northwestern China

Besides geometrical and velocity effects, two global parameters may disturb the comparison between acoustic logs recorded in a borehole and those derived from seismic data: (a) the amplitude spectrum of the basic seismic wavelet, which controls the power of resolution; and (b) its phase characteristics that monitor the resemblance. The latter is of major importance in stratigraphic interpretation of seismic impedance logs (recognition of geologic transitions). The most important causes of distortions can sometimes be determined and compensated. The seismic source signature needs to be recorded, analyzed, and processed to determine the seismic wavelet. Phase distortions due to recording equipment (geophones and laboratory) are very important, and need to be corrected. Trav l into the earth introduces multiples and absorption. Multiples can be dealt with in the classic way with a few precautions. Then an estimation of the amplitude spectrum of signal and noise at a given time-depth provides one with a quantitative estimate of the best achievable resolution of the actual data, and the absorption curve in amplitude spectrum using the recorded source spectrum. A good match between experiments and theory leads to the estimated impulse response of absorption at a given time, and reversely, to an optimized (eventually time-varying) correction operator. Further conventional deconvolution processes should not be used. Nevertheless, when distortions cannot be corrected, and if well data are available, a deconvolution program controlling both amplitude and phase can be used interactively to achieve an equivalent result. End_of_Article - Last_Page 980------------

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J. M. Komatitsch

Papers

How to Correct Seismic Data to Improve Seismic Impedance Logs: ABSTRACT