Research On Sequence Stratigraphy And Potential Lithologic Traps In Putaogou, Tuha Basin

Putaogou, with a total area of 350km2, is located in the west of Taipei Depression, Tuha Basin. In total, 22 wells had previously been drilled. Of these, 15 had been drilled for exploration of structural traps and were dry holes. The seven remaining wells had been drilled for exploration of lithologic traps and five were found to be commercially viable. Tuha Petroleum Company Geoscientists decided that the strategic focus for further exploration should change from structural traps to lithologic traps. The purpose of this research project was to explore the lithologic petroleum traps in Putaogou. The main challenge was that the depositional systems, reservoirs and zones of potential were not clearly defined, leading to exploratory inefficiency. An early consideration in this project was that two wells drilled on the lithologic traps in 2003 (Huobei-1 and Susha-1) were dry. Both theory and technology played a vital part in solving these difficulties. Specifically, non-marine sequence stratigraphy, sedimentology and petroleum geology were applied using well logging, cutting log, core, seismic and testing technologies. The methodologies and findings were as follows: ⑴Using seismic-well correlation and visualization technology, the distribution of the 3D sequence framework was reconstructed. Five three-order sequences are defined in Jurassic (J) and Cretaceous (K). It was determined that the LST and TST of Sq4 (J2q) should be the exploratory targets. During J2q, the paleo-depositional setting is a flexing slope break-belt. During J2s, the paleo-depositional setting is a moderate slope break-belt. During K, the paleo-depositional settings are flexing slope break-belt and steep slope break-belt. It was discovered that the slope break-belt that developed in the foreland is different to that in a faulted basin where the gradient of a break-belt is abrupt. ⑵Well logs and seismic attributes were used to recover the sediment micro-facies of J2q. It was discovered that the depositional system of J2q is controlled by the slope break-belt and faults. Above the break and to the north of P401 fault, a fan delta formed. Above the break and to the south of P401 fault, a delta formed. On the slope break-belt, three lowstand fans formed. Beneath the slope break-belt, the sublucstrine fans were deposited. The sediment sources of J2s, J2q and K originate from the same directions. The types of depositional systems within each are quite different, due to the scale of their sediment sources. ⑶In researching the relationship between oil reservoirs and system tracts, it was found that oil and gas tends firstly toward TST and secondly toward LST. The depositional systems include bars, lowstand fans and sublucstrine fans. It is proposed that future development focus on the (braided) delta above the break, the lowstand fans on the slope belt and the sublacstrine fan beneath the slope break-belt that was uplifted by Himalaya movement. ⑷It was determined that Spontaneous Potential (SP) is fully indicative of reservoirs,...