Identification of astronomical cycles in mixed fine-grained sedimentary rocks and its application in fine stratigraphic division and correlation - a case study of the fourth member of Shahejie Formation in the Leijia area, the western sag of the Liaohe Depression

Key words: 
• Mixed fine-grained sedimentary rock /
• Astronomical cycle /
• Sedimentation rate /
• Fine stratigraphic division /
• Western sag

Abstract: [Objective]The lacustrine fine-grained sedimentary rock mixed with clay, felsic, carbonate and analcite minerals develops in the fourth member of Shahejie Formation of the Leijia area in the western sag of the Liaohe Depression in the Bohai Bay Basin and is the main carrier for the occurrence of oil and gas. Due to the complex composition and rapid lateral changes of mixed fine-grained rocks, the reservoirs are highly heterogeneous, which brings certain difficulties to the prediction of high-quality reservoirs. [Methods] Taking the Lei 15 well, the Lei 14 well and the Lei 61 well in the Leijia area of the western sag as an example, based on time series analysis method, high-precision carbonate U-Pb dating and natural gamma logging data, the mixed fine-grained rocks of the the fourth member of Shahejie Formation were analyzed by cyclostratigraphy. [Results and Discussions] 1) The optimal sedimentation rates of Well Lei 15, Well Lei 14 and Well Lei 61 were estimated by COCO, and it was found that the optimal sedimentation rates increased sequentially and were 10.57 cm/kyr, 11.4 cm/kyr and 13.93 cm respectively; Perform spectrum analysis on the paleoclimate proxy indicator (GR) and compare it with the data spectrum analysis results of the standard ETP curve (eccentricity, slope, precession composite curve), identify astronomical cycle signals in mixed fine-grained rocks in Wells Lei 15, Lei 14 and Lei 61, And use the 405 kyr long eccentricity for astronomical tuning, and use the age 43.4±1.7 Ma at 2766.61m in Well Lei 14 as the anchor point to establish an absolute astronomical time scale. [Conclusions] By conducting cyclostratigraphic research on the fourth member of Shahejie Formation in the Leijia area, astronomical cycle signals in the fourth member of Shahejie Formation can be effectively identified. This method quantitatively establishes a fine stratigraphic division and comparison framework with time attributes, which plays an important role in guiding further oil and gas exploration in the area and broadens the applicability of cycle stratigraphy in the Bohai Bay Basin.