Geochemistry and origin of Ordovician oils in the Rewapu Block of the Halahatang Oilfield(NW China)

The oils in the Rewapu Block of the Halahatang Oilfield in the Tarim Basin(northwestern China) were geochemically analyzed by gas chromatography–mass spectrometry to investigate their origin and possible secondary alteration. The physical properties and chemical compositions of these Ordovician oils vary greatly, ranging from light to extremely heavy oils. All these oils belong to one single population and were derived from same source kitchen/bed. The differences in their chemical compositions and physical properties mainly result from secondary alteration of oils that were charged during the early phase of trap formation. The thickness of Upper Ordovician and Silurian caprocks of oil reservoirs in the Rewapu Block is higher than that in the north part of the Halahatang region, and therefore, significant biodegradation process is prevented. Compared to heavy oils in the north part, the Rewapu oils are dominated by light oils with no 25-norhopanes and no GC–MS ‘‘hump''(UCM: unresolved complex mixture) on the chromatographic baselines. The heavy oils in Wells RP7 and RP101 C were primarily influenced by water washing since apparent reduction of alkylated dibenzothiophenes was observed. The local geological background favored the water washing alteration. The reservoir geochemical study has practical application in hydrocarbon exploration and predicts the nature of oils in this oilfield.

川中地区侏罗系天然气与原油轻烃地球化学特征对比

天然气伴生凝析油及轻质油的轻烃地球化学参数常常被用于天然气成因和油—气对比研究,但蒸发分馏等次生作用对天然气和伴生原油的轻烃组成会产生一定的影响。基于川中地区16件侏罗系天然气与伴生凝析油或轻质油样品气相色谱分析,探讨了轻烃组成及相关地球化学参数在天然气和原油中的差异。研究表明:蒸发分馏作用对油气轻烃组成影响明显,相对于伴生原油,天然气轻烃组成具有高异构烷烃和正构烷烃相对含量,而贫环烷烃和芳香烃的特征。蒸发分馏作用对甲基环己烷指数、环己烷指数、Mango参数K1等成因参数影响不大,伴生原油的这类指标均可以较好地反映天然气的成因类型。蒸发分馏作用对成熟度参数异庚烷值影响较小,而对庚烷值、2,4-DMC5/2,3-DMC5值影响明显。轻烃化合物沸点的不同可能是导致天然气与原油中不同类型化合物相对含量存在差异的主要原因,并且沸点较低的化合物在天然气中相对含量更高。组成轻烃参数的化合物的沸点差异越大,该参数受蒸发分馏作用影响越大,天然气与伴生原油的该参数值相差也越显著。而当比值参数中组成分子与分母的化合物的沸点之差小于2℃时,该参数基本不受蒸发分馏作用的影响。