渤海海域秦南凹陷东南缘断层活动对古近系油气富集的控制

FAULT ACTIVITIES ON THE SOUTHEASTERN MARGIN OF THE QINNAN SAG IN THE BOHAI SEA AND THEIR CONTROL OVER PALEOGENE HYDROCARBON ACCUMULATIONS

秦南凹陷东南缘陡坡带沙一二段储层中发现了大量油气,但油气富集的控制因素不清,严重制约了该区的进一步勘探和评价。利用钻井、测井、三维地震和分析化验等资料,结合沉积背景,运用断层活动速率法定量分析了秦南凹陷东南缘断层的活动特征,探讨了断层活动性与古近系油气富集的关系。研究表明:研究区古近纪至现今,断层活动呈"弱-最强-弱-强-最弱-弱"的特点。沙三段、东三段和东二段沉积期,断层活动较强,为主要的烃源岩和盖层发育期,断层活动越强的位置,烃源岩厚度越大、成熟度越高;沙一二段、东一段至现今沉积期,断层活动弱,为主要的储层发育期,断层活动弱的位置以及调节带储层较发育。断层活动弱的位置,易形成岩性圈闭;断层活动强的位置,易形成断层-岩性圈闭。古近纪时断层纵向上的强弱演化为油气成藏提供了优质的烃源岩和储、盖层条件。东营组沉积末期,烃源岩成熟并开始运移,断层幕式活动活化了断层带,为油气在纵向和横向上的运移创造了有利的通道条件。明化镇组上段沉积期,油气进入储层并开始成藏,断层活动直接影响着油气的保存和再分配,由于盖层厚度较大且分布广,成藏后断层活动减弱,进而阻止了油气的再分配,使得油气主要在沙一二段储层中富集,形成现今亿吨级大油田。

A substantial amount of oil and gas accumulations has been discovered in Es12 on the steep slope of the southeastern margin of the Qinnan Sag. However, the factors which control the enrichment of oil and gas remain unknown. The situation severely restricted the further exploration and evaluation of oil and gas resources in the region. Based on the data of drilling, logging, 3 D seismic and analytical tests, combined with the study of depositional background, the method of fault activity rate was adopted in this paper to quantitatively study the fault characteristics. The relationship between fault activities and Paleogene oil and gas accumulations was also discussed. It is found that from Paleogene to the present, fault activities in the study area is characterized by a "weak-strongest-weak-strong-weakest-weak" pattern with time. The periods of Es3, Ed3 and Ed2, while fault activities were strong, are the main periods for source rock and cap rock deposition. In the area, stronger the fault activity, thicker the source rocks and higher the maturity. During the periods from the first and second Members of the Sha 1 and Dong 1 Formations, fault activities are weak in the region, and that is the time for reservoir formation. The period from Es12 and Ed1 to the present is the time for reservoir development, while fault activities are weak. Reservoirs were more developed in the accommodation zones. In the places with weak fault activities, it is easier for lithological traps to form, while for the area with strong fault activities, fault-lithologic traps are more favored. In Paleogene, the alternation of strong and weak fault activities with time provided favorable conditions for the formation of high quality source rocks and reservoir cap systems. By the end of the Dongying Formation, the source rocks are matured and oil and gas started migrating. The episodic fault activities reactivated the fault zone and favorable channels were available for the vertical and horizontal migration of oil and gas. During the late period of the Minghuazhen Formation, oil and gas moved into the reservoir and began to accumulate. Fault activities directly affect the preservation and redistribution of oil and gas. Fortunately, they were weakened after the oil and gas accumulation, that led to the formation and preservation of billion tons large oilfields.