Oil and gas exploration in eastern part of the Tarim basin was quite successful recently with several commercial gas accumulations being discoved in high to over-matured source region. Yingnan2 (YN2) gasfield, situated in Yingnan structure of the Yingjisu depression, is one representative gas accumulation. To study oil and gas genetic characteristics and accumulation history in this region attracts wide interests to geologists and geochemists. YN2 gases are rich in nitrogen and relatively wet. The methane and ethane stable carbon isotopic compositions are in the range of -38.6‰~-36.2‰ and -30.9‰~-34.7‰, respectively. They are characterized by marine source origin but more matured than variety of other marine originated oil-type gases discovered in the Tarim basin so far. Biomarkers from their associated condensates indicate closely affinity with the Lower Paleozoic Cambrian-Lower Ordovician marine source rocks rather than the shallower Middle Jurassic coal measures. Both gas and associated condensate were obviously derived from the Cambrian-Lower Ordovician marine source rocks in the Yingjisu depression, however, to understand oil and gas generation and accumulation histories remains puzzling. The Cambrian-Lower Ordovician marine source rocks are over-matured (VRE=3%~4%) at present time and their burial and maturity evolution histories indicate that paleo-geotemperature increases rapidly from 90℃ to 210℃ within 20 Ma (458~438 Ma) during the late Caledonian period (Middle-Late Ordovician) due to rapid subsidence and quick heating, which leads source rocks experience very short “oil window” (about 10 Ma) and incomplete oil expulsion before gas generation stage. They can be regard as “dead source rocks” with no oil and gas generation potential by the end of Ordovician. On the other hand, fluid inclusion homogenization temperature, burial and hydrocarbon generation histories suggest that YN2 gas in the Jurassic reservoir was formed within recent 10 Ma. It is not possible for the Cambrian-Lower Ordovician source rocks to generate wet gas with dryness coefficient of 0.82~0.90 during this period, whereas the Middle Jurassic strata did not mature enough (R_O< 0.7%) to form such large quantity of oil and gas. Such obvious discrepancy between oil and gas generation, accumulation histories and their properties may reflect special accumulation processes occur in this region. High concentration of diamondoid hydrocarbons detected from condensates may indicate that oil had experienced extensively thermal cracking. Very heavy isotopic composition in whole oil is a supplemental oil cracking evidence. Gas isotope curves illustrate that gas formation temperature is above 190℃. All these evidences suggest that gases discovered in the Tadong area are primarily form by oil cracking, which may turn into the vital exploration target in this region. Oil and gas accumulation processes can be reconstructed as following: the Lower Paleozoic Cambrian-Lower Ordovician marine source rocks experienced oil peak-wet gas-kerogen cracking dry gas stages during the late Caledonian movement, oil and gas migrated upward along faults to the Middle-Upper Ordovician or Silurian sandstone reservoirs to form ancient oil and gas accumulations. With increasing burial depth during the late Yanshanian movement, these deep buried ancient oil accumulations start to cracking and large quantity of cracked gas migrates upward. Since incomplete trap development and poor tightness of top seal at this time, methane and other light components were preferentially diffusion and leaking away, leading obvious compositional fractionation in gas accumulation process and concentrated wet gas components. With trap tightness getting better and continuous gas charging in the late Himalayan movement, gases filling into the traps and leakage away through faults reach a dynamic status in certain scale, which finally forms deep sourced secondary condensate accumulations in the study area.