Nenjiang Formation 1 member Ichnocoenosis and its deposit environment of Upper Cretaceous from Yaonan 5 well, Songliao Basin

This paper is the result of oil and gas exploration engineering.

Objective Abundant trace fossils were preserved in Upper Cretaceous strata in Songliao Basin, but related studies mainly focus on the northern part of the Songliao Basin. The sedimentary environment of section 1 of the Nenjiang Formation was restored by trace fossils identification in the southern part of the Songliao Basin, and this paper will provide evidence to support subsequent trace fossil studies in this area.

Methods According toobserve wells of Yaonan 5 drill core of section 1 of the Nenjiang Formation, we identified 2 ichnogenus and 2 ichnospecies including Chondrites and Planolites, and established Chondrites-Planolites ichnocoenosis. Combining with ichnofossils surrounding rock, burrow scale, record of well logging curve to analyse trace maker habit; According well of Yaonan 5 sampling, measured sample contents of Uranium, Thorium, Vanadium, Nickel and Cobalt; Using U/Th, V/(V + Ni) and Ni/Co methods to restore trace fossil preservation strata paleo-oxygen facies of section 1 of the Nenjiang Formation.

Results This study suggests that Chondrites-Planolites ichnocoenosis were formed in deep water environment with weak water energy; Paleo-oxigenation facies data analysis suggests that method of U/Th indicated oxic environment, methods ofV/(V + Ni) and Ni/Co indicated dysaerobic-anaerobic environment.

Conclusions The trace fossils were preserved in mudstone in well Yaonan 5 with small burrow diameter scale and low abundance, indicating the water was anoxic. Combined with the paleooxygen facies data and logging curve analysis, considering that tracemaker were living in semi-deep lake which with low energy and poor oxygen content.