A Study On The Subtle Reservoir Of The Lower Tertiary And Its Forming Mechanism In Bonan Sag

Bonan sag is a secondary negative structural unit in Jiyang depression. Structural evolution of the sag can be divided into four developing stages, i.e. thrusting structure stage of Triassic, negative inversion stage of latter Jurassic to early Eocene, faulted basin stage of middle Eocene, and depression basin stage from Miocene. The faulted basin stage is the most important forming stage of reservoir rock and source rock in the sag. Controlled by cycle activity of structure in faulted basin stage in Bonan sag, there sediment evolution stages can be identified in Eocene, such as Ek Sequence, Es4 Sequence, Es3 Sequence and Es2-Ed Sequence, and faces of underwater fan of alongshore, fan delta, braided stream delta, lakefront, etang, medium to deep lake, salt lake, and turbidite sand etc., were developed in the cycles. Structure activity controlled the distribution and characteristics of sediments by controlling the palaeogeomorphology of sedimentation.Lithologic reservoir and composite reservoir associated with it are the most important subtle reservoirs in bonan sag, which can be mainly divided into up dip pinch out reservoir of sand, lenticle reservoir of sand, channel sand reservoir, fault- lithologic reservoir. Faces of underwater fan of alongshore, deltas, lakefront, etang, bioherm and turbidite sand are advantageous forming sites of reservoir rock. Higher organic carbon content in deep burial strata provides good conditions for the forming of organic acid, and also the developing of secondly porosity. The sutble reservoir in Bonan sag is formed by tertiary migration of oil,oil-bearing serires contact directly with source rock provides advantage for the gathering of oil, faults activated in latter Ed to Ng also provided good channel for oil migration and gathering form Es3 and Es4 source rock upward to reservoir rocks.Dip pinch out reservoir of sand tends to distribute in the area of glutenite fan, lenticle reservoir of sand mainly locates in arenaceous distal turbidite region, and channel sand reservoir usually develops in channels of glutenite fan and channels surrounding uplift.