地球物理学报  2015, Vol. 58 Issue (10): 3660-3670   PDF    
川西坳陷鸭子河地区基于多种古温标的钻井热史恢复
朱传庆1,2, 邱楠生1,2, 江强1,2, 胡圣标3, 张硕1,2    
1. 中国石油大学(北京) 油气资源与探测国家重点实验室, 北京 102249;
2. 中国石油大学(北京) 地球科学学院, 北京 102249;
3. 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029
摘要:利用镜质体反射率(Ro)、磷灰石裂变径迹(AFT)和伊利石结晶度(IC指数)等古温标恢复了四川盆地川西坳陷的钻井热史,对比了不同温标最高古地温的恢复结果.研究表明,研究区晚白垩世至今总体表现为冷却及抬升剥蚀的过程,地温梯度由约26 ℃·km-1降低至约22 ℃·km-1,剥蚀量约1.3~1.9 km.约80 Ma以来开始抬升剥蚀, 40—2.5 Ma经历了一个热平静期,第四纪存在一定的增温,地温梯度增高约5 ℃·km-1.三种古地温恢复结果具有较高的一致性,相对于镜质体反射率(Ro)和磷灰石裂变径迹(AFT)等成熟古温标,伊利石结晶度作为有机质成熟度指标和沉积岩古温标的应用处于定性分析阶段,该指标的热演化模型仍需进一步探索.
关键词镜质体反射率     磷灰石裂变径迹     伊利石结晶度     热史恢复     川西坳陷    
Thermal history reconstruction based on multiple paleo-thermal records of the Yazihe area, western Sichuan depression
ZHU Chuan-Qing1,2, QIU Nan-Sheng1,2, JIANG Qiang1,2, HU Sheng-Biao3, ZHANG Shuo1,2    
1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;
2. College of Geosciences, China University of Petroleum, Beijing 102249, China;
3. State Kay Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Abstract: Thermal history is significative to the geodynamics and petroleum geology of a sedimentary basin, for temperature is important for many geodynamic and the hydrocarbon accumulation process, especially the maturation evolution of hydrocarbon source rocks and the history of hydrocarbon generation.In the western Sichuan Basin, a continental depression is superimposed on the Paleozoic craton. The Paleozoic source rocks in this area have experienced a multi-stage and complex structural-thermal evolution and thus require different methods and effective geothermal indicators to reconstruct the thermal history,which will help discover the equivalence between different geothermal indicators and establish a set of geothermal indicators for the thermal history reconstruction of the area.
The thermal history of a basin is usually reconstructed using various paleo-temperature indicators, including vitrinite reflectance (Ro), apatite fission track (AFT), (U-Th)/He in apatite and zircon, reflectance of bitumen and vitrinite-like macerals, the hydrogen index, and fluid inclusions. Ro and AFT are the most widely used indicators and the related modeling methods are relatively well established, other indicators such asillitecry stallinity (IC), Raman spectroscopy, and acoustic rock emissions have also been studied and used in the quantitative reconstruction of paleo-temperatures.In this paper, the thermal history of the western Sichuan depression was reconstructed based on three geothermal indicators: Ro, AFT and the IC index, using the paleo-temperature gradient method, Monte Carlo modeling and the estimation of metamorphic temperature stage respectively, and the maximum paleo-temperatures reconstructed based on different indicators were compared.
The thermal history reconstruction of CY92 using the paleo-temperature gradient method shows that the maximum paleo-temperature gradient of this borehole was ~26 ℃·km-1, the maximum paleo-heat flow was ~60 mW·m-2, both was larger than the present. The thickness of removed sediments on the surface of the unconformity between the Upper Jurassic and the Cenozoic was approximately 1900 m. This indicates that the area has undergone an overall, continuing structural-thermal evolution of cooling, uplifting, and denudation since the rock strata reached the maximum temperatures.AFT thermal history modeling result of CY92-1 reveals that the basin had experienced subsidence between 220 and 140 Ma and a thermal quiet period between 145 and 80 Ma, crustal uplift and erosion occurred in this area between 80 and 40 Ma, but after another thermal quiet period from 40 to ~2.5 Ma, the temperatures of the strata began to rise. The thickness of the removed sediments was estimated to be ~1900 m, which is generally consistent with the Ro analysis results.In borehole CY95, at a depth of ~4080 m, the KI was 0.60 (°Δ2θ), and the maximum paleo-temperature was ~150 ℃; at a depth of ~6000 m, the KI was 0.42(°Δ2θ), and the maximum paleo-temperature was ~200 ℃. The paleo-temperature profile of CY95 indicates a less of ~690 m in the thickness of removed sediments.
In conclusion, the structural-thermal evolution in the western Sichuan depression since the late cretaceous to present is denudating and cooling, the geothermal gradient reduced from about 26 ℃·km-1 to about 22 ℃·km-1, the erosion thickness is about 1.3~1.9 km. The uplifted and eroded is continuous since ~80 Ma to 40 Ma, a thermal quiet period kept from 40~2.5 Ma, then the geothermal gradient raised about 5 ℃·km-1 since about 2.5 Ma. There was high consistency between the paleo-temperatures of the western Sichuan depression that were reconstructed based on Ro, AFT, and KI. Thus, these three indicators can be used as paleo-temperature indicators for the thermal history reconstruction of the area. However, relative to Ro and AFT, the illitecry stallinity index is still controversial and needs more research for using it as a maturity and geothermal indicator in the sedimentary rocks.
Key words: Vitrinite reflectance     Apatite fission track     Illite crystallinity index     Thermal history reconstruction     Western Sichuan depression    
1 引言

沉积盆地热历史是指在地质历史中盆地热状态的变化,包括了热流、地温梯度以及地层温度的演化等,是地球动力学和石油地质学研究中的重要内容(邱楠生等,2005).温度是诸多地球动力学过程的驱动力(Morgan,1984;Wang,1996),是油气成藏的重要因素之一,尤其对烃源岩有机质成熟度演化和生烃史影响重大(Tissot and Welte,1984;Tissot et al.,1987; 邱楠生等,2004).在盆地尺度上,热史主要通过各种古温标,如镜质体反射率(Ro)、磷灰石裂变径迹(AFT)、磷灰石和锆石(U-Th)/He、沥青和镜状体反射率、氢指数、流体包裹体等来重建(胡圣标等,1998;邱楠生等,2004;秦建中等,2009a;2009b).各种古温标中,应用最为普遍、模拟方法相对成熟的是镜质体反射率(Ro)和磷灰石裂变径迹(AFT).近年来,伊利石结晶度、激光拉曼光谱、岩石声发射等在古地温的定量恢复中也有了较成功的探索和应用(徐春华等,2005;王河锦等,2007;秦建中等,2009a;2009b;李佳薇等,2011;Liu et al.,2013).

中国大陆地区海相盆地形成时代早(古生代或更早),经多期次构造叠加与改造,其长期和复杂的构造-热演化过程决定了其热史的多期性和复杂性,而且其上覆陆相盆地或陆相地质时期热史与海相时期热史的演化特征、恢复方法和有效古温标亦有所差别.在以四川、塔里木等为代表的海陆相叠合盆地中,古生代海相地层往往是油气的重要烃源层,发育时代老,热演化程度高,且缺乏镜质体等传统古温标和磷灰石样品,其有机质成熟度的准确标定和热史恢复一直是一个难题(秦建中等,2009b).因此,这类海陆叠合盆地的热史恢复,除要求多样化、且彼此补充的热史恢复方法体系(胡圣标等,2008)外,更需要探索有效的古温标,并对不同温标之间的等效性进行研究,进而建立起可系统应用于多期热史恢复的古温标组合.本文利用镜质体反射率(Ro)、磷灰石裂变径迹(AFT)和伊利石结晶度(IC指数)恢复了四川盆地川西坳陷的钻井热史,并对比了不同温标最高古地温的恢复结果,对于分析不同古温标之间的等效性、建立适合于研究区热史恢复的古温标组合具有一定意义.

2 地质背景

川西坳陷(图 1)位于四川盆地西部,西以龙门山前大断裂为界,东以龙泉山断裂为界,面积约为5×104 km2(陈冬霞等,2010),是在古生代克拉通盆地之上发展起来的前陆盆地.晚三叠世印支运动前,川西位于上扬子克拉通盆地西缘,整体为一西倾的碳酸盐台地(马永生等,2009),印支期,随着龙门山、大巴山造山带的形成(刘树根等,2009;李智武等,2011),海水退出,进入陆相沉积阶段,大量碎屑物质进入川西,沉积形成了巨厚的以岩屑砂岩为主的上三叠统(陈杨等,2011),此后进入前陆盆地演化阶段.川西地区前陆盆地演化阶段的构造-沉积特征已有较多的研究(郭正吾等,1996;陶晓风,1999;刘和甫等,2006; 刘树根等,2009;2011;刘金华等,2010;李智武等,2011;Tian et al.,2013),由于龙门山自印支期以来向川西坳陷多次逆冲推覆,在产生区域断裂的同时也使盆山结合部位地层抬升剥蚀,喜山期随着印度与欧亚大陆发生碰撞,在龙门山区激起了更为强烈的逆冲运动,导致龙门山逆冲带及川西前陆盆地的定型(王天泽,1997;赵建成等,2011).由研究区川鸭92井的埋藏史恢复(图 2)可以看出,陆相沉积初期,晚三叠世沉积速率较大;早侏罗世经历了抬升剥蚀和沉积间断,造成下侏罗统的沉积厚度较薄;中侏罗世-晚白垩世,研究区接受沉积;白垩纪晚期开始持续的抬升剥蚀;新生代晚期(主要为第四纪)接受了少量的沉积.

图 1 川西坳陷地质概况及地层柱状图(据赵建成等,2011;张永刚等,2007修改)Fig. 1 Geological sketch map and the stratigraphic column of the western Sichuan depression(modified from Zhao et al.,2011; Zhang et al.,2007)

图 2 川西坳陷鸭子河地区川鸭92井地层埋藏史(a)和地质历史时期沉积剥蚀速率(b)Fig. 2 Strata buried history(a) and the rate of deposition and denudation of the borehole CY92 in the Yazihe area,western Sichuan depression

鸭子河构造位于川西坳陷中南段,为龙门山推覆构造带中段前缘关口断裂下盘的半背斜构造,形成于喜马拉雅晚期,受控于关口和彭县断层,为一北东向断背斜(朱彤和梁恩宇,2001; 刘大成等,2002).根据生储盖发育情况,该区的含油气系统可以分为上三叠统-上侏罗统的上部陆相含油气系统和中三叠统及下伏地层构成的下部海相含油气系统(张永刚等,2007;赵建成等,2011).鸭子河构造的钻井,如川鸭92井、川鸭95井等,主要钻遇地层为上侏罗统至上三叠统的陆相层系(图 1),上三叠统煤系地层烃源岩包括煤、炭质页岩、灰-黑色泥岩和粉砂质泥岩,烃源岩较发育,以烃源岩为主的马鞍塘组、小塘子组、须三段和须五段的暗色泥岩厚度达几百米,须二段和须四段储层段内部也发育有机质丰度较高烃源岩的夹层(杨克明等,2003;陈冬霞等,2010).须家河组砂岩和侏罗系以陆内冲积扇、河、湖三角洲相沉积层构成该区陆相含油气系统的主要储集层.须三段、须五段、上侏罗统遂宁组泥、页岩作为上三叠统的区域盖层具有厚度大、突破压力高和分布稳定的特征,白垩系内的膏、盐层可作为侏罗系的直接盖层(汪泽成等,2002).

3 古温标数据及热史恢复 3.1 镜质体反射率热史恢复

镜质体是一种煤素质,主要由芳香稠环化合物组成.随热成熟的增加,镜质体分子结构中的芳香环缩合度增大,形成更加密集的结构单元,从而使反射率增大(陈义才等,2007). Ro值的演化主要与有机质受热温度和时间有关,且以温度为主.随着镜质体反射率演化的热动力学机制研究的深入,利用Ro 恢复古地温的方法也由早期的图版法( Hood and et al.,1975; Cooper,1977)、TTI拟合法(Lopatin,1971; Waples,1980; Lerche et al.,1984)发展为干酪根热降解的化学动力学模型方法(Antia and David,1986; Braun and Burnham,1987; Armagnecl et al.,1989;Burnham et al.,19871989; Burnham and Sweeney,1989; Sweeney and Burnham,1990),其中,Sweeney和Burnham(1990)建立的EASY Ro%模型在目前的热史恢复中应用较广.

川鸭92井Ro数据主要集中在2000~5000 m的三叠系,Ro值主要在0.8%~2.0%之间,显示出随深度而增大的变化趋势.基于EASY Ro%模型,采用古地温梯度法(Duddy et al.,1991;胡圣标等,1998;邱楠生等,2004)恢复了川鸭92井的最高古地温(图 3).