| Continental rift basin is a very important kind of petroliferous basins.They are generally composed of several halfgraben sags shaped as dustpans,containing a steep slope belt,a trough belt and a gentle slope belt.Geographically,the gentle belts are adjacent to the high-quality source rocks in the trough belts,and usually located in the low-potential areas of hydrocarbon migration.In addition,various traps are widely distributed in the gentle belts,which are very important regions for hydrocarbon exploration in rift basins.However,due to the complex geological characteristics,the hydrocarbon accumulation rules in the gentle slope belts are extremely complex.As a result,it is difficult to determine the predonderant hydrocarbon accumulation areas,which restricts the further exploration and development of hydrocarbons.The Bodong Sag is a typical halfgraben in the eastern Bozhong sub-basin,the Bohai Bay Basin.Its eastern gentle slope belt shows great inhomogeneity in tectonic characteristics and hydrocarbon distribution,which is a good case to study the complex hydrocarbon accumulation rules in the gentle slope belts of continental rift basins.Based on seismic,tectonic,sedimentary,logging and testing data,the basic geological conditions for hydrocarbon accumulation,such as source rocks,reservoirs,cap rocks,traps,migration pathways in the gentle slope belt of the Bodong Sag were clarified.By comprehensively comparing the tectonic characteristics of different regions in the gentle slope belt,suitable indexes were selected to classify the gentle slope belt,and then the differences of hydrocarbon accumulation conditions in different types of gentle slope belts were analyzed and summarized.Oil-source correlation was proceeded by using biomarkers to identify the sources of oils in different structures,and the stages and timing of hydrocarbon accumulation in the gentle slope belt were determined by using fluid inclusion testing data.Lateral migration distance of crude oils was determined by comparing the maturity of crude oils and source rocks,and based on the detailed analysis of the hydrocarbon accumulation rules in typical wells and the spatio-temporal coupling relationships of each hydrocarbon accumulation condition,the hydrocarbon accumulation models of different types of gentle slope belts were established.The main controlling factors of hydrocarbon accumulation in the gentle slope belt of the Bodong Sag were analyzed from the distribution of high-quality source rocks,the distribution and transportation efficiency of tectonic ridges,and the transporting and sealing effects of faults on hydrocarbons.On the basis of the above,the distribution rules of hydrocarbons in the gentle slope belt were further revealed and summarized.Main conclusions in this study are as follows:(1)Different regions in the gentle slope belt of the Bodong Sag vary in geometry,tectonic evolution history and fault combination characteristics,which all have an obvious effect on hydrocarbon accumulation.Therefore,according to the above three indicators,the gentle slope belt of the Bodong Sag was firstly divided into three types from south to north: the broad-steep inherited gentle slope belt,the broad-gentle modified gentle slope belt and the narrow-steep inherited gentle slope belt.(2)There are great distinctions in hydrocarbon accumulation conditions among different types of gentle slope belts in the Bodong Sag.Long tectonic ridges and vast fault-related traps exist in the broad-steep inherited gentle slope belt,and large-scale high-quality source rocks were developed in its adjacent west sag.Short tectonic ridges and abundant fault-related traps exist in the broad-gentle modified gentle slope belt,and large-scale and relative large-scale high-quality source rocks were developed in its adjacent west sag and the residual sags in the gentle slope belt.However,long tectonic ridges characterized by low hydrocarbon transporting efficiency and relatively less fault-related traps exist in the narrow-steep inherited genlte slope belt,and the poor-quality source rocks were developed in its adjacent northwest sag.(3)Owing to the differences in the hydrocarbon accumulation conditions,different types of gentle slope belts in the Bodong Sag show different hydrocarbon accumulation models.Hydrocarbons started to accumulate in the gentle slope belt of the Bodong Sag since 8.8 Ma.A combined fault-ridge-unconformity conduction,distant/near-source hydrocarbon accumulation model was proposed in the broad-steep inherited gentle slope belt.A vertical fault conduction,near-source hydrocarbon accumulation model and lateral/vertical fault conduction,distant-source hydrocarbon accumulation model existed in the broad-gentle modified gentle slope belt.A combined fault-ridge-unconformity conduction,distant-source hydrocarbon accumulation model was identified in the narrow-steep inherited slope belt.(4)The main factors controlling hydrocarbon accumulation in the gentle slope belt of the Bodong Sag are the distribution of high-quality source rocks,the distribution and transporting capacity of faults and tectonic ridges,and the sealing ability of faults.The distribution of the high-quality source rocks controlled the regional differences of hydrocarbon accumulation.Faults and tectonic ridges are the main hydrocarbon migration paths controlling hydrocarbon accumulation.The distribution of tectonic ridges and fault zones controlled the plane distribution of hydrocarbon accumulation,and the vertical transport capacity of faults controlled the horizon distribution of hydrocarbon accumulation.The sealing ability of faults controlled whether hydrocarbons can accumulate in traps or not.(5)The gentle slope belt of the Bodong Sag has the hydrocarbon distribution rules of “two-level and two-dimension joint control”.The first level is the zonation level and the controlling factor is the gentle slope belt type.The corresponding “two dimensions” are the distribution of high-quality source rocks and dominant migration paths respectively.The second level is the trap level,and the controlling factor is the direct control effects of faults on hydrocarbons accumulation.The corresponding“two dimensions” are the transporting capacity and sealing ability of faults respectively.The above results clearly reveal the hydrocarbon distribution rules in the gentle slope belt of the Bodong Sag,which can provide a good guidance for the hydrocarbon exploration in the gentle slope belt,and can also provide a favorable reference for the study of hydrocarbon distribution rules in the gentle slope belts of other continental rift basins. |
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