Abstract:

A systematic analysis was carried out on a combination of field outcrop data with surface natural gamma ray measurement and element contents （Fe， Ca， Mg and B） as well as well data to characterize the sequence stratigraphy of the third member of Bashijiqike Formation in an outcrop section from the northern Kuqa Depression. The results show that the member has three distinct lithologic intervals， with the lower interval characterized by two sedimentary cycles （periods） of thick brownish-gray medium to fine sandstone and thick brown mudstone， the middle interval by a large set of thick brown and brown-gray siltstone， fine sandstone with thin reddish brown mudstone interbeds and the upper interval by alternating brown-gray siltstone， fine sandstone and reddish-brown mudstone. The surface natural gamma curves values range between 5 and 40 API and occur mostly as dented bell curves or flattened boxes with dents in between， which can be readily compared with well logs. The variation trends of Fe³⁺ and Fe²⁺， Ca/Mg ratio and B in the section show striking stratigraphic cycles， corresponding to the paleo-water variation in an order of shallow - deep - shallow - deep - shallow - deep - shallow for the Bashijiqike Formation. These long-period cycle interfaces in the third member of Bashijiqike Formation are contrastive and can be easily identified in outcrops. Observation of the lithology， color， bedding and other characteristics of the outcrop section reveals two types of short-term base-level cycle superposition patterns， i.e.， the “upward-deepening asymmetric” pattern and the “upward-deepening followed by upward-shallowing near-symmetric” pattern； and if combined with the characteristics of natural gamma curves and elemental analysis， medium-term cycles can also be identified. Based on this method， the third member of Bashijiqike Formation in the outcrop can be divided into three medium-term cycles （fourth-order sequences） and six short-term cycles （fifth-order sequences）， with one short-term cycle missing from parts of the outcrop. However， compared with data gathered from wells drilled closer to lake basin center， absence of one medium-term cycle and four short-term cycles are often observed in the outcrop. The medium-term cycles of MSC1， MSC2 and MSC3 in the outcrop correspond respectively to the medium-term cycles of MSC2， MSC3 and MSC4 in subsurface， and they share the similar alternating patterns of sand body with mudstone interlayers/barriers. The study may be used to guide gas exploration in the deep of northern Kuqa Depression.

Key words: sedimentary cycle, high-resolution stratigraphy, Bashijiqike Formation, outcrop profile, deep gas, Cretaceous, Dabei Gasfield, Kuqa Depression, Tarim Basin