Abstract:

On the basis of comprehensive analysis data on outcrop,seismic reflection profiles and drilling,the Youquanzi oil-bearing structure is confirmed to be an asymmetric half-box anticline retained by conjugate kink zones with a straight hinge dipping towards NNE.The northern limb is a large scale kink zone of over 800 m wide and dips towards SSW at a high angle with sub-vertical inner-bed in it.The rotation of the inner-beds indicates that the southern side shears reversely towards the north.Four small-scale kink zones are developed on the southern limb and shear reversely southward,carving the limb into the step occurrence dipping SSW at 20? as a whole.Along the boundaries of the kink zones on both limbs,there are merely a few small-scale high-angle reverse faults.Fractures,mainly bedding-separation fractures,followed by separation fractures sub-normal to the bedding(mostly occurring in competent calcareous siltstones)and occasionally small-scale low-angle thrusts,are developed in the kink zones.These fractures are commonly filled with fiber gypsum and show locally some oil traces,indicating that these fractures are the products of high-pressure fluid.Seeing that the kink zones and relevant anticlinal highs are fractured and favorable for both oil-gas migration and accumulation,they can be a new way for finding fissure-type reservoirs with the absence of enough ideal lithologic-re-servoirs.

Key words: half-box fold, kink zone, fissure-type reservoir, Youquanzi structure, Qaidam basin