| The unveiling of the Bozhong 19-6 gas field has reframed the traditional conception of the Bohai Bay Basin as principally oil-dense with restricted gas assets.Recently,the Qingshui Sag and its surrounding areas within the Liaohe Subbasin of the Bohai Bay Basin have undergone significant progress in natural gas exploration.However,the intricate nature of the discovered gas reservoirs,characterized by a variety of genesis types and uneven distribution patterns,has introduced notable challenges to further exploration and development initiatives within the Qingshui Sag.This issue can be traced back to three primary reasons:(1)The intricate tectonic evolution and sedimentary systems in the rift basin have led to a highly heterogeneous distribution and characteristics of source rocks,with existing studies only delivering a relatively generalized understanding of these properties.(2)Natural gas reservoirs can have various origins,including biogenic,thermogenic,and mixed sources.When the analysis of source rocks doesn’t align with the diverse origins of the gas,it can lead to multiple interpretations of the gas source.(3)The phenomenon could also be attributed to a lack of comprehensive and systematic studies on the dynamic processes of hydrocarbon generation,migration,accumulation,and the key factors influencing these processes in the Qingshui Sag and its surrounding areas.Without a detailed understanding of these processes and factors,it can be challenging to explain the observed variations in natural gas sources and distributions.The goals of this study are to(1)leverage geochemical experimental outcomes and geophysical prediction methodologies to accurately ascertain the geochemical properties and spatial distribution of hydrocarbon source rocks of varying facies types within the sedimentary-layered framework of the Qingshui Sag;(2)categorize types of natural gas and their correlation with effective gas source rocks,using parameters such as natural gas isotopes and maturity,while taking into account variations in structural positions;(3)construct an extensive three-dimensional geological model of the Qingshui Sag by employing advanced modeling and facies-control techniques,with the intent to accurately define the spatial distribution and volume of effective gas source rocks;(4)perform simulations and analyses of the dynamic processes relating to the generation,migration,and accumulation of natural gas,using the aforementioned findings,to pinpoint favorable zones for future exploration endeavors.This research unveils the existence of fan-delta,sub-lacustrine fan,and lacustrine depositional systems within the Western Depression.The overall total organic carbon(TOC)content within the research zone suggests the presence of promising hydrocarbon source rocks,with the deep-lake mudstone demonstrating superior hydrocarbon source rock properties.The study solidifies a strong linkage between types of organic matter and depositional facies.Mudstones in the fan-delta,sub-lacustrine fan,and shallow-lake from the western slope zone are classified as Type II organic matter.The deep-lake mudstone,however,is categorized as Type I organic matter.The nearshore subaqueous fan and rift shoulder-sourced fan-delta present characteristics of Type III organic matter.The planar distribution of mudstones,along with their geochemical characteristics,is precisely delineated by amalgamating the spatial distribution of depositional facies with mudstone thickness and the type/abundance of organic matter.The natural gas in the Western Depression of the Liaohe Subbasin comprises five types of natural gas:mixed gas of primary biogenic and thermogenic origin,biogenic gas modified by thermal processes or mixed with biogenic methane,oil-type gas,coal-type gas,and mixed gas.Based on the origin of natural gas,gas source rocks are further classified into humic gas source rocks,sapropelic gas source rocks,and primary biogenic gas source rocks.The study identifies the southern part of the Qingshui Sag as the source of coal-type gas,originating from the Fourth Mbr.of the Shahejie Formation,while the oil-type gas primarily originates from the lower sub-member and middle sub-member of the Shahejie Formation.The mixed gas is contributed by effective sapropelic gas source rocks and humic gas source rocks in the Fourth Mbr.and the Third Mbr.of the Shahejie Formation.In the northern part of the Qingshui Sag,the coal-type gas primarily originates from the third Mbr.of the Shahejie Formation,while the mixed gas is contributed by effective sapropelic gas source rocks and humic gas source rocks in both the third and fourth Mbrs.of the Shahejie Formation.This study underscores the feasibility of utilizing geological statistics to simulate the abundance and type of organic matter in hydrocarbon source rocks.Established three-dimensional basin modeling results suggest a distinction in the distribution of gas source rocks between the third and fourth Mbrs.of the Shahejie Formation;the southern portion of the Fourth Mbr.predominantly harbors humic gas source rocks,whereas the northern portion mainly contains sapropelic gas source rocks.The opposite pattern is observed in the third Mbr.The study quantifies the volume of gas source rocks in the research area,approximating a total volume of 586.4 km~3of effective gas source rocks,comprising113.4 km~3of primary biogenic gas source rocks,232.9 km~3of sapropelic gas source rocks,and 240.1 km~3of humic gas source rocks.Furthermore,the study recognizes the upper to the middle sub-member of the third Mbr.as the principal gas-bearing intervals.This study integrates geochemical parameters and indicators of natural gas and gas source rocks,alongside dynamic simulations of gas migration and accumulation(1D,2D,and 3D),to elucidate the control of different gas source rock types on the origin of natural gas reservoirs within and surrounding the Qingshui Sag.The activities of the Dongying Fm.faults have significantly modified pre-existing gas reservoirs,leading to late-stage accumulation.Cap rock development has played a crucial role in preserving the gas.The Qingshui Sag exhibits distinct characteristics including multiple sources,single-phase hydrocarbon generation,and late-stage accumulation.Three petroleum accumulation units are identified within the study area:the Third Mbr.of the Shahejie in the Shuangtaizi tectonic belt and its adjacent western slope,the Xinglongtai-Qingshui Sag,and the remaining areas of the Qingshui Sag and western slope.Three accumulation models are summarized as follows:single sag-multiple sources-fault zone,single sag-multiple sources-slope zone,and dual sag-multiple sources-fault zone.Using 3D simulations,the study calculates significant volumes of natural gas in specific intervals,such as the upper submember of the Fourth Mbr.of the Shahejie Fm.,the lower and the middle submember of the Third Mbr of the Shahejie Fm.,with volumes reaching 12,677×10~8 m~3,9,996×10~8m~3,and 14,399×10~8 m~3,respectively.These intervals are identified as high-quality natural gas source rock layers.Additionally,the lithological gas reservoirs within the Third Mbr of the Shahejie Fm.and the natural gas reservoirs within the Fourth Mbr.,which are sealed by high-pressure lithological gas reservoirs,represent potential gas resources within the Qingshui Sag.This study’s findings not only enhance our scientific understanding of geologically complex regions within the study area but also provide crucial decision support for future resource exploration and development.This aids in more accurately assessing the resource potential of different areas and,based on this,formulating more targeted exploration and development strategies. |
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