Abstract:

 Largescale lacustrine beachbar oil pools have been discovered recently in Dongying Depression, Shandong Province. The beachbar sequences in the upper fourth member (with an age of about 45 Ma to 42 Ma) of the Eocene Shahejie Formation (Ess4) became the important exploration targets in this area. However, distributions and deposit environments of beachbars were poorly studied. Lake water depth is an important control factor for deposition and distribution of beachbars. A micropaleontological study was carried out with the objective to obtain quantitative paleowater depths during the deposit periods of the Ess4 in Dongying Depression. Early Middle Eocene microfossils (ostracodes, microphytoplankton and benthic algae) from 47 wells were analyzed. A method of determining quantitative water depths by using of multimicrofossil proxies was developed. As a result, the early Middle Eocene paleobathymetric map illustrated with 2 m isobath for Dongying Depression was reconstructed. The results showed that Dongying Depression was generally a shallow to deep lake during that period, with water depth ranging from 0 m along its coast to more than 30 m in the central area (Well Haoke 1). Three features were seen from the variation of water depths. (1) Northern area is characterized by deeper waters, steeper and narrower lake slopes, whereas flatter and wider beach terraces covered by shallower waters occur in the southern region. (2) Steep and narrow lake slopes are clearly related to the headlands of highlands on lake coasts. (3) Three water zones with NESW extending direction were recognized: a small shallow lake zone (about 6 m) from Well Lai 9 to Well Wang 18 in the northerneast corner of Dongying Depression, the largest and deepest zone in the central, and a very shallow terrace bar (<3.5 m) occurred in the southwest area. These features were well correlative to abundant sedimentary data and evidences from previous studies. For example, delta deposits founded in the northern deep waters and beachbar occurred in the southern areas. All correlations suggested that the multimicrofossil proxies for quantitative water depths are reliable. The paleobathymetric map with relative high resolution provided possibility to understand and depict smallscale deposit areas for shallow water beachbars and deep water deltas. This study also suggested that microfossil study could still serve as a useful tool for updated oil exploration, however, good microfossil records with high time resolution combined with high resolution seismic and sequence analysis are needed.

Key words: paleowater depth, microfossils, beachbars, the lowstand systems tract, Ess4, Eocene, Dongying Depression