Geochemical Characteristics of Ordovician Natural Gas and Its Applications in the Division of Hydrocarbon Plays in Sichuan Basin

Abstract

In Sichuan Basin, the exploration of the Ordovician is in a lower degree and the main gas reservoirs are upper Ordovician Baota Fm.(O3bt). and lower Ordovician Tongzi Fm.(O1tz). But the origins, genesis and distribution of natural gas in the Ordovician are unclear. This article is to understand the geochemical characteristics of Ordovician gas and apply them to divide hydrocarbon plays and predict the favorable exploration areas. An integrated study method is put forward in this article. First, summarize the geochemical characteristics of Ordovician natural gas by analysing the composition and carbon isotope data of gas samples. Second, analyse the origins, genesis of natural gas by comparing with natural gas from other formations of Sichuan Basin, natural gas generated from type III kerogen or type II kerogen in other basins. Third, divide the hydrocarbon plays by the above-mentioned results and source-reservoir-seal assemblages studies. Finally, analysing the accumulating process of Ordovician gas to predict the favorable exploration areas. The following conclusions are made. 1) The compositions and gas dryness coefficient of nature gas from both O3bt Fm. and O1tz Fm. show different sources and different gas maturity, which amounts of gas in O3bt reservoirs are from the source rocks of Silurian Longmaxi Fm. with relatively low gas maturity, while the natural gas in O1tz reservoirs are from the source rocks of Cambrian Qiongzhusi Fms. with relatively high gas maturity. 2) Large amounts of natural gas from both O3bt and O1tz Fms. are genetically crude oil cracking gas, which generated from the type II kerogen. 3) The accumulating process of hydrocarbon in Ordovician reservoirs is divided into three phases, namely the end of Silurian is the mature stage of oil generated from the source rocks, and the end of the Triassic is the high mature stage of natural gas generation and the stage of amounts of oil cracking gas in the past oil reservoirs, and the early Cretaceous period is the stage of readjustion and redistribution of natural gas. 4) Two hydrocarbon plays are proposed existing in Ordovician, namely the lower play (O1tz Fm.) with lower-source and upper-reservior indirectly and the upper play (O3bt Fm.) with upper-source and lower-reservoir directly, showing their favorable areas are respectively in the surrounding areas of the paleo-uplift and the fracture-pore type reservoir belts with favorable tectonic backgrounds.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015