Discovery of hiatus in Feixianguan Formation and its geological implications, Sichuan Basin, SW China

Authors: YANG YU, XIUCHENG TAN, PEIYUAN CHEN, HUITING YANG, TENG MA, JIAN CAO, XIUJU JIN

Abstract: The lower Triassic Feixianguan Formation of the Sichuan Basin is an important exploration target for marine natural gas in China. It is widely believed that a certain interval from Fei 1 to Fei 4 displays conformable contacts rather than long-term wide-ranging hiatus. However, 2 denudational interfaces on the top of Member Fei 1 and Fei 2 respectively were found in the present work, which relies on the delicate core observation of 4 cored wells in Puguang Gas Field, microscopic observations of microsamples in terms of thin sections, scanning electron microscopy and cathodoluminescence, and geochemical testing. Evidence indicates that on the top of the first and second Member of Feixianguan Formation there existed karst unconformities, which is in distinguished contrast to previous interpretations of conformable contacts. According to those geological and geochemical investigations, a new classification is proposed for the interior Feixianguan Formation. The tectonic episode division of Indosinian movement consequently may be modified from the previous conception of a first-order episode to 3 sub-first-order episodes that correspond to the impact of 2 uplifting processes during the early Indosinian orogeny. These uplifting processes are the cause in interpreting the pervasive exposure and unconformable interfaces in Fei 1 and Fei 2 karstifications. Thus, we suggest another appropriate division of 3 third-order sequences that may suit the Feixianguan Formation, which correspond to Fei 1, Fei 2, and Fei 3-Fei 4. Regional reservoir formation was also largely affected by penecontemporaneous karstification. Massive dissolution pores fill with seepage sediments but generally remain effective reservoir spaces.

Keywords: Hiatus, unconformity, penecontemporaneous karst, sequence interface, carbonate reservoir, Feixianguan Formation, Puguang Gas Field, Sichuan Basin

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