Sequence Stratigraphy of the Quaternary Qigequan Group, Qaidam Basin, Western China

Abstract

The Qaidam Basin is a Mesozoic—Cenozoic compresso-shearing basin in western China that has been subsiding since the Indosinian orogeny; subsidence was a strong inherited feature in its Quaternary development. Several natural gas fields have been found in the Quaternary strata since 1958; this world-famous area has total reserves of more than 300 billion cubic meters. However, specific targets are problematic for further gas exploitation in the basin. Based on comprehensive methods, including drilling/logging, seismic and Milankovitch cycles, this paper offers a high-resolution sequence stratigraphical study of the Quaternary Qigequan group, and identifies further exploration targets in the Qaidam Basin. Based on more than 60 sets of well-logging information and reservoir inversion data for 30 seismic sections, six sequence boundaries were identified in the Qigequan group. Characterizations of the sequences were summarized and divided into five third-order sequences (sq1, sq2, sq3, sq4, and sq5, respectively). In the Qaidam Basin, delta, fan delta, and lacustrine facies have developed in the Qigequan group. The main frequency of the Tainan-5 well is 0.026, 0.046, and 0.058Hz, and conversion of the frequency inverse into corresponding wavelength is 38.4, 21.7, and 17.2 m, which then gives the deposition time for each main frequency cycle as 42, 24, and 19 ka, respectively. This is similar to the 41 ka period of slope nutation of the Milankovitch cycle. It is also similar to the 23 ka and 19 ka precession cycles, which means that from the Quaternary to the present, Milankovitch cycles have strongly influenced depositon in the Qaidam Basin. Delta-front sandstone and sandstone lens pinchouts in the upper half cycle of the basal level of e sequence 2 (sq2) appears to represent a profitable target for further exploration.