Diagenetic Sequence in Altered Volcanics and its Impact on Hydrocarbon Reservoirs: A Case from Lower Permian in the Ke-Xia Area of Junggar Basin, China

Shifa, Zhu; Shifa, Zhu

Volcanic and volcaniclastic rocks are significant hydrocarbon reservoir rocks. In these rocks, low-temperature alteration (<200 degree) of volcanic materials is universal and improves reservoir quality greatly.

On the basis of observations of available cores, thin-sections and SEM, together with analysises of rock geochemistry, this paper is aimed to study on characteristics and origins of authigenic minerals as well as their impacts on pore spaces in the volcanics-bearing hydrocarbon reservoirs, and to discuss diagenetic evolution in altered volcanics, especially chloritization, zeolitization, albitization, and calcilization, to clear the control of volcanics alteration on reservoirs quality and provide a scientific geological foundation for quality evulation and prediction.

Mafic volcanic materials of the lower Permian in northwestern margin of Junggar Basin underwent devitrification, chloritization (clayzation), zeolitization, albitizaiton, and calcilization (carbonation). These processes of diagenetic alteration were controlled by various factors such as the intermediate-mafic host rock, the temperature (85 ~160 degree) and pressure (especially tectonically compressive stress), and the chemistry of the fluid phase (e.g. pH and cation ratios).

The analysis of porosity data based on cores indicates that a small amount of diagenetic cement, like pore-lining Fe-rich chlorite can influence the mechanical properties that control sediment strength, inhibiting deformation and late cementation and preserving porosity. Acidulous water dissolves zeolite, albite, and calcite that precipitated in early-stage alkaline conditions, and dissolved pores are created. The establishment of diagenetic evolution during the volcanics alteration can be favor of prediction of good reservoir. Compared with mafic volcanic rocks, intermediate-silicic lavas and pyroclastic rocks without tight chlorite cementation may have better porosity. Influenced by the NaHCO3 type formation water and the organic acid recharged by hydrocarbon source rocks, coarse conglomerate with soluble analcime cements has developed many secondary dissolved pores, and is the favorable clastic reservoir.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013