Cyclostratigraphy And Its Implications For Petroleum Geology In The Paleogene Lacustrine Strata From The Nanxiang Basin

The Nanxiang Basin which is a Mesozoic-Cenozoic intermontane basin developed since Cretaceous period with multiple genesis mechanisms and multi-stage rifting is a hydrocarbon-rich depression located at the Qinling-Dabie orogenic belt and the northern margin of the Yangtze platform in central China.Previous scholars did tremendous work about fundamental geology in the Nanxiang Basin,however,the chronostratigraphy of the Paleogene succession of the Basin remains controversial due to the lack of an accurate geological time scale.In this work,the study focuses on the B270 and BS1 boreholes from depocenters in the Biyang Depression of the Nanxiang Basin and establishes a high-precision timescale by selecting a reliable paleoclimate proxy and performing cyclostratigraphic analysis for these two holes.With the help of high-resolution geochemical data and microscopic thin-section observations from the BYHF1 borehole,this study could detect the Milankovitch cycles in sedimentary archives.At the same time,this research will be conducted via cross discipline between cyclostratigraphy and petroleum geology to explore the astronomical response of hydrocarbon source rocks,and to explore the potential relationship between astronomical cycles and stratigraphic sequences.The main research results are as follows:1.This study shows that the Milankovitch cycles(eccentricity,obliquity and precession)are well preserved in the Paleocene strata of the Biyang Depression.In this paper,we take 405-kyr long-eccentricity cycles as the main period for astronomical tuning and establish a 43.2-Myr floating astronomical time scale(ATS).The Paleogene/Neogene boundary in the Biyang Depression(the top boundary of the Liaozhuang Formation)which is a major tectonic interface with less denudation in the depocenter of the Biyang Depression can be used as an anchor point for calculation.The floating ATS was then anchored to this anchor point.The Cretaceous/Paleocene boundary in the Biyang Depression(the bottom boundary of the Yuhuangding Formation)which is another major tectonic interface with less denudation in the depocenter of the Biyang Depression can be used as a scrutinized point for calculation.In addition,the bottom boundary of the 1st member of the Hetaoyuan Formation which is the Eocene/Oligocene boundary can be another scrutinized point.2.Microscopically,through a comprehensive analysis of geochemical elements from the core section of the 3rd member of the Hetaoyuan Formation(Eh₃)in BYHF1 borehole,combined with microscopic thin section observation,this study suggests that the sedimentary varves consisting of alternating light-and dark-colored layers may represent annual varves,controlled by precession.Macroscopically,after analyzing the cyclostratigraphy of high-resolution gamma-ray(GR)logs and Total organic carbon data from Eh₃ in B270 borehole,we identify 1.2-Myr long-period obliquity amplitude imprint that may have influenced hydrocarbon source rock development by affecting water depth and,in turn,medium-quality hydrocarbon source rocks.3.For Eh₃ in B270 borehole,we subdivide third-order stratigraphic sequences by the1.2-Myr long-period obliquity cycles,fourth-order stratigraphic sequences by the 405-kyr eccentricity cycles and fifth-order stratigraphic sequences by 100-kyr eccentricity cycles.In this study,we identify 8 third-order stratigraphic sequences,?25 fifth-order stratigraphic sequences and?98 fifth-order stratigraphic sequences.4.The ATS enables the assignment of the ages to non-marine biozones in our study area.5.There are major tectonic events or unconformities that began at?50 Ma in the Cenozoic terrestrial rift lacustrine basin of east China.In our study,we find that?50 Ma tectonic event in the Biyang Depression may be influenced by a combination of factors such as the change in the convergence direction of the Pacific plate with East China,the slowing of that rate of convergence,and perhaps far-field stresses caused by the collision of the India Plate with Eurasia.