The Research Of Hydrocarbon Accumulation Conditions Of Buried Hill In Tanan Depression

Tanan depression is an important oil producing sag of Hailer-Tamtsag Basin, and themain oil producing formation concentrates upon the first member of Nantun formation atpresent. But with the low level understanding of deep buried hill, it is necessary of reinforcingthe studies of buried and seeking the favorable exploration blocks, not only for the stabledevelopment of the oil field, but also for raising the output of production. Based on all kindsof the existing information, we studied the typical buried hill and then analysised the formingconditions of buried hill reservoir in Tanan depression. On the condition, the accumulationconditions of buried hill were discussed, and the favorable district of buried hill reservoir waspredicted ultimately.By the study of Tanan depression, the buried hill is classifled as three types: syngeneticdomed mountain, syngenetic block mountain, syngenetic horst mountain. It is concluded thatthere are five fault systems related to the buried hill type. With the less distribution, Earlyfoundational fault makes little difference; Early strike-slip faults (Ⅰ) and Mid-term stretchfracture(Ⅱ) that makes less destruction and more regulation and blocking effect, are beneficalto hydrocarbon accumulation and preservation of oil and gas; Early strike-slip—Mid-termstretLate tenso-shear axial-direction（Ⅰ-Ⅱ-Ⅲ），Early strike-slip—Mid-term stretch—Latetenso-shear axial-direction—Later reverse fault（Ⅰ-Ⅱ-Ⅲ-Ⅳ）plays an important role ofdestruction and migration, so the buried hill with the considerable development of this fault isadverse to the oil and gas preservation generally.The reservoir rock types are mainly as sedimentary rock (including pyroclastic rocks andclastic rocks), and there are also some metamorphic rocks of magmatic rocks. Themeasurement of porosity and permeability indicates that reservoir rock is low porosity, lowpermeability reservoir. The basic kinds of accumulation space include pores, caves, as well asfractures. There develops a wide range of regional dark mudstone in the reservoir, where thehigh and middle of the first member of Nantun formation (Special lithologic formation) aswell as the second member of Nantun formation acts as the main hydrocarbon source rockssection. The cap formation with medium sealing ability which has the largest area and bettersealing ability distributes in the most part of depression. The traps of reservoir have largequantity and area, wide distribution range. And many traps develop in the buried hill and themiddle structural zone. The migration pathway consisting of source-fault, surface ofunconformity and connected sand bodies ensures the connection of oil source and reservoir in the buried hill.The main controlling factors of the formation of the buried hill reservoir in theresearched area, which mainly formed in the late depositional stage of the Yimin group,contain the window for source rock providing hydrocarbon, the migration pathway of oil andthe reservoir bed. And there are two patterns in the oil accumulation of the buried hill in theresearched area. The first pattern is that the oil migrates laterally through source-fault to thereservoir of buried hill in the base rock and accumulates, and the second one is that the oilmigrates laterally through surface of unconformity to the reservoir of buried hill andaccumulates. The author predicted the favorable place for prospecting the buried hill reservoirby folding and analyzing comparatively the research results which came from the researchesin the various sides for buried hill accumulation.