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Multi-stage Hydrocarbon Accumulation and Adjustments and its Prediction of Favorable Areas for Oil and Gas Exploration in the Lower Paleozoic Formation of Tarim Basin

Xiongqi Pang1,2, Zhenxue Jiang1,3, Caifu Xiang1,2, Hong Pang1,2, and Junqing Chen1,3
¹State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China
²Basin and Reservoir Research Center, China University of Petroleum, Beijing, China
³Unconventional Natural Gas Research Institute, China University of Petroleum, Beijing, China

The lower Paleozoic formation in Tarim basin experienced multistage tectonic events and hydrocarbon accumulation, and these reservoirs suffered adjustments, reconstruction and destruction due to later tectonic movements. As a result, the rule for distribution of these reservoirs was very complex. It was of great significance to improve the efficacy of oil and gas exploration in Tarim basin by studying the multi-stage hydrocarbon accumulation and adjustments as well as the prediction of favorable areas for oil and gas exploration in lower Paleozoic formation of Tarim basin. Studies showed that combination of functional elements, including the source rock (S), depositional facies (D), regional caprock (C) and low potential zone (X), controlled the formation and distribution of reservoirs in lower Paleozoic formation of Tarim basin. Their orderly vertical combination (C/D/X/S) controlled favorable formation for hydrocarbon accumulation; their horizontal superimposing (C∩D∩X∩S) controlled favorable areas for hydrocarbon accumulation; their timely simultaneous association (TC=TD=TM=TS) decided the time for hydrocarbon accumulation. Later superimposing of tectonic movements led to the adjustment, reconstruction, and destruction of earlier reservoirs in lower Paleozoic formation of Tarim basin. The basic geological model for reconstruction of reservoirs caused by superimposing of tectonic events included destruction from superimposing of intense-intense tectonic movements, reconstruction from superimposing of intense-weak tectonic movements, and protection from superimposing of weak-weak tectonic movements. Amounts of destroyed hydrocarbons were mainly controlled by the intensity, sequence of tectonic movements, sealing ability of regional caprocks on hydrocarbons as well as original accumulation amounts of hydrocarbons. On the basis of these investigations, the quantitative model for the relationship between destroyed hydrocarbons, together with remaining resources potential, and these controlling factors was established, which provided a new method to evaluate the amounts of destroyed hydrocarbons in superimposed basins. Later combination of facies-potential-source decided the petroliferous property of Lower Paleozoic traps in Tarim basins. Despite various times for hydrocarbon accumulations occurred in Lower Paleozoic of Tarim basin, the last hydrocarbon accumulation was most significant for oil and gas exploration. Although there were some tectonic movements in Lower Paleozoic layers of Tarim basin, controlling effects of the most recent tectonic movements was the most crucial. The multi-stage hydrocarbon accumulation areas and protection areas by the weak-weak superimposing effects were most favorable for oil and gas exploration. Traps that developed in the near-source path, favorable reservoirs and low potential areas of these areas had a high probability for hydrocarbon accumulation. The hydrocarbon accumulation model of near source-favorable facies-low potential could be used to predict and evaluate the petroliferous property of most favorable exploration targets, thus preferably deciding the drilling targets. Exploration of hydrocarbons in lower Paleozoic formation of Tarim basin consists of four procedures. First of all, on the basis of controlling effects of geological threshold on hydrocarbons, figure out the amounts of hydrocarbon generation and loss of each migration and accumulation unit, then predict favorable resource areas based on material balance theory. Secondly, identify functional elements, restore its evolution history, and investigate their controlling effects on hydrocarbons in favorable resource areas, then predict the boundary, area and probability for hydrocarbon accumulation of multi-stage superimposing by virtue of the controlling model of functional elements on reservoirs’ distribution. Thirdly, study the evolution history of the basin and investigate the adjustment, reconstruction and destruction of reservoirs, then predict favorable exploration areas with a larger remaining resource potential in favorable areas for hydrocarbon accumulation according to the reconstructing model of tectonic movement on reservoirs. Finally, investigate the accumulation effects in favorable exploration areas, and decide most favorable drilling targets based on the near source-favorable facies-low potential accumulation model. By applying new theory and technology, the most favorable resource areas, most favorable accumulation areas and most favorable exploration areas were predicted. The results showed that 100% of the reservoirs discovered in lower Paleozoic layers of Tarim basin were distributed in the most favorable exploration areas as predicted. By the end of 2009, 100% of the 122 successful wells out of 296 wells drilled in lower Paleozoic formation of Tarim basin were distributed in the most favorable exploration areas as theoretically predicted. Among these wells, 95% of wells whose daily oil and gas equivalent excelled 18 tons were with the Facies-Potential-Source Index over 0.6. Among 174 wells with no hydrocarbons, 24% were due to bad combination of functional elements, 19% were caused by the destruction from tectonic movements, and 57% were due to bad combination of facies-potential-source. The theory “controlling effects on hydrocarbon accumulation of the combination of elements-reconstruction by superimposing of tectonic movements-identification by later combination of facies-potential-source” had been well applied in the exploration practice of hydrocarbons of lower Paleozoic formation in Tarim basin. In terms of 21 most favorable exploration areas predicted and evaluated in Tazhong uplift, 100% had been proved by drilling wells and achieved commercial production. They provided theoretical and technological foundation for annual 285 million tons newly discovered and annual 18% growth of oil and gas reserves of Tarim Oilfield Company in recent years.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013