Jianfa Han

Control of hydrocarbon accumulation by Lower Paleozoic cap rocks in the Tazhong Low Rise, Central Uplift, Tarim Basin, West China

Despite the absence of regional cap rocks in the Lower Paleozoic for the entire Tazhong Low Rise, several sets of effective local cap rocks are well preserved on the Northern Slope. Of these the best is the Ordovician mudstone of the Sangtamu Formation; the second is the Silurian Red Mudstone Member of the Tatairtag Formation and the marl of the Ordovician Lianglitag Formation; and the third is the gray mudstone of the Silurian Kepingtag Formation. The dense limestone of the Ordovician Yingshan Formation and the gypsum of the Middle Cambrian have shown initial sealing capacity. These effective cap rocks are closely related to the distribution of Lower Palaeozoic hydrocarbons in the Tazhong Low Rise. With well-preserved Sangtamu Formation mudstone and its location close to migration pathways, rich Lower Paleozoic hydrocarbon accumulation can be found on the Northern Slope. Vertically, most of the reserves are distributed below the Sangtamu Formation mudstone; areally, hydrocarbons are mainly found in the areas with well-developed Sangtamu Formation mudstone and Lianglitag Formation marl. Burial history and hydrocarbon charging history show that the evolution of Lower Palaeozoic cap rocks controlled the accumulation of hydrocarbon in the Tazhong Low Rise. Take the Red Mudstone Member of the Tatairtag Formation and Sangtamu Formation mudstone for examples: 1) In the hydrocarbon charging time of the Late Caledonian — Early Hercynian, with top surfaces at burial depths of over 1,100 m, the cap rocks were able to seal oil and gas; 2) During the intense uplifting of the Devonian, the cap rocks with top surfaces at burial depths of 200–800 m and 500–1,100 m respectively were denuded in local areas, thus hydrocarbons trapped in earlier time were degraded to widespread bitumen; 3) In the hydrocarbon charging time of the Late Hercynian and Himalayan, the top surfaces of the cap rocks were at burial depths of over 2,000 m without intense uplifting and denudation thereafter, so trapped hydrocarbons were preserved. Based on cap rocks, the Ordovician Penglaiba Formation and Lower Cambrian dolomite could be potential targets for exploration on the Tazhong Northern Slope, and combined with hydrocarbon migration, less risk would be involved.

3D Seismic attribute optimization technology and application for dissolution caved carbonate reservoir prediction

Summary The fractured and caved Ordovician carbonate reservoirs in Tarim Basin are characterized by significant seismic anisotropy and heterogeneity. It is very difficult to predict this type of reservoir using common seismic attribute analysis technology because of the uncertainty of analysis results. On the basis of investigating the application condition and effectiveness of various seismic attributes, focusing on the heterogeneity of carbonate reservoir and taking logging data as the known training samples, this paper realizes the 3D optimization processing for multi seismic attribute data volume by combing of multi statistic methods such as multivariate discriminant analysis, kernel principal component analysis etc. The established reservoir discriminant model in this paper has the total coincidence rate of 90.13% by retrospective test, proving the validity of the model. The optimization results of multi seismic attribute data can finely sculpture the favorable distribution range and geometric shape of dissolution caved carbonate reservoir in 3D space, thus obtaining good geologic results. It is expected that this paper can provide a scientific and practical method for seismic multi-attribute optimization and dissolution caved carbonate reservoir prediction.

A new reservoir prediction method: PCA value-weighted attribute optimization

Summary This paper introduces the principle component attribute (PCA) technology into carbonate reservoir prediction, and a PCA value-weighted attribute optimization method is proposed here. As the calculation time-window of the PCA valueweighted attribute is the same (viz., free of the constraint of time-window scale), it can effectively avoid the negative effect of different time-windows on attribute optimization results. A test area is chosen from Tarim Basin, and the reservoir information of the known wells is treated as the training samples. The discriminant model of PCA valueweighted attribute and reservoir parameters are built. Through the retrospective test, the total fit rate between the attribute optimization results and the production results is up to 94.62%, proving that the discriminant capability of PCA value-weighted attribute for reservoir quality is significant. Finally, the proposed method is employed for the integration prediction of carbonate reservoir in test area. The prediction results finely exhibiting the distribution of favorable carbonate reservoir. This paper is aimed at providing a scientific and practical method for seismic attribute analysis and reservoir prediction.

P-wave Fracture Prediction Noise Attenuation Algorithm Using Pre-stack Data With Limited Azimuthal Distribution: A Case Study In Tazhong 45 Area, Tarim Basin

Seismic reflection amplitude at the top of a fractured layer varies as a function of both offset and azimuth (AVOZ). In this paper, a special inversion algorithm based on P-wave AVOZ is presented for fracture detection. Conventionally, wide azimuth data is required to conduct AVOZ inversion. This is because the existence of noise makes the solution unstable. In this paper a fracture prediction method applicable to data with narrow (limited) azimuth distribution is introduced, where a noise attenuation algorithm is employed in course of the inversion. The inversion algorithm is then applied to field data in Tazhong 45 area where data was acquired under limited azimuthal distribution. The inverted results (fracture density and orientation) were further examined by wells with FMI and core data, indicating that the inversion algorithm presented in this paper is feasible for fracture prediction. This research implies that it is possible to make use of old ‘narrow’ (limited) azimuth data and also potentially save some cost for future acquisition.