准噶尔盆地南缘齐古油田油气成藏再认识及勘探启示

Recognition of hydrocarbon accumulation in Qigu oilfield and implications for exploration in the southern margin of Junggar Basin

准噶尔盆地南缘白垩系之下的下组合勘探潜力大,但目前勘探发现率低。齐古油田位于准噶尔盆地南缘山前断褶带,发现于1957年,2011年在齐古背斜西侧低部位钻探的齐古1井获工业气流,为齐古油田带来了新生命,给久攻不克的准噶尔盆地南缘下组合勘探增强了信心。对齐古油田复杂油气成藏过程的再认识有助于厘清准噶尔盆地南缘下组合的成藏条件与有利勘探方向。通过分析齐古背斜构造模型及构造演化,结合油气的分布及地球化学特征,综合利用油气源对比、流体包裹体观察和测温、盆地模拟、泥岩盖层有效性评价等技术和方法,详细分析了齐古油田多层系和多期油气成藏过程与运移聚集规律。研究结果表明,齐古北断裂为晚侏罗世—新近纪长期继承性活动断裂,对齐古构造的形成与油气的运移和聚集都起到重要的作用。齐古地区在晚侏罗世末期发育古构造,晚新生代以来的冲断褶曲作用形成现今的齐古背斜,而古构造发育区则在构造掀斜作用下成为现今背斜的北翼。'早油晚气'的成藏过程导致的蒸发/运移分馏作用使得残留原油密度大、含蜡量高。受喜马拉雅晚期强烈冲断作用影响,齐古北断裂沟通下盘深埋的侏罗系烃源岩生成的高成熟煤成气进入齐古背斜形成聚集,但在不同部位,由于多期油气的充注过程和晚新生代构造抬升对盖层保存条件的破坏程度存在差异,齐古地区总体上形成'上油下气'的异常油气分布格局。齐古油田成藏过程的再认识对勘探有着重要启示:在齐古背斜北翼的古构造发育区找早期残留油、西北倾伏端构造低部位找气;齐古北断裂下盘侏罗系的原地冲断构造、古近系紫泥泉子组的冲断构造为潜在勘探领域。齐古油田高成熟煤成气的发现证实准噶尔盆地南缘下组合的侏罗系烃源岩规模有效生气,指示在靠近齐古油田的乌奎背斜带中段,下组合的天然气勘探潜力大。

The lower assemblage under the Cretaceous in the southern margin of Junggar Basin has large exploration potential,but low exploration discovery rate at present.Qigu oilfield,located in the piedmont fault-fold belt of the southern margin of Junggar Basin,was discovered in1957.In2011,that the industrial gas flow yielded in Well Qigu-1 drilled in the west of the Qigu anticline brought a new life to Qigu oilfield,and strengthened confidence in exploring the lower assemblage in the southern margin of Junggar.Reconsidering the complex oil and gas accumulation process of Qigu oilfield helps to clarify the reservoir forming conditions and favorable exploration direction of lower assemblage in the southern margin of Junggar Basin.By analyzing the structural model,tectonic evolution,hydrocarbon distribution and geochemical characteristics of Qigu anticline,and comprehensively utilizing the techniques and methods such as oil-gas source correlation,fluid inclusion observation and temperature measurement,basin modeling,effectiveness evaluation of mudstone caprock,this paper analyzes the multilayer and multi-stage hydrocarbon accumulation process and pattern of Qigu oilfield in detail.The results show that the northern Qigu fault,a long-term inherited active fault from Late Jurassic to Neogene,plays an important role in the formation of Qigu structure and the migration and accumulation of oil and gas.At the end of the Late Jurassic period,paleostructure was developed in the Qigu area.Since the Late Cenozoic,the thrust folds have been developed into the current Qigu anticline,while the paleostructure development area became the north limb of the present anticline under the action of tectonic tilt.The evaporation/migration fractionation caused by the'early oil and late gas'accumulation process led to the high density of residual crude oil and high wax content.Influenced by the strong thrust in the late Himalayas,the high-mature coal-derived gases generated by the Jurassic source rocks deeply buried in the footwall passed through the northern Qigu fault to accumulate in the Qigu anticline.However,in various parts,due to the different destruction to the caprock preservation conditions caused by the multi-phase oil and gas charging and the Late Cenozoic tectonic uplift,the Qigu area generally presents an abnormal oil and gas distribution pattern of'oil in the upper and gas in the lower'.Reconsidering the reservoir-forming process of Qigu oilfield has important implications for exploration,which the paleostructure of the north limb of Qigu anticline is the favorable area for the early residual oil exploration,the lower part of the northwest-plunging end is the area for gas exploration,and the Jurassic in-situ thrust structure in the footwall of the northern Qigu fault and the thrust structure of the Paleogene Ziniquanzi Formation are the potential areas for oil and gas exploration.The discovery of high-mature coal-derived gas in Qigu oilfield confirmed that the Jurassic source rocks in the southern margin of Junggar Basin effectively generated gases in a large scale,indicating that in the middle section of Wukui anticline near the Qigu oilfield,the exploration potential of natural gas in the lower assemblage is quite large.