Possible seismic hydrocarbon indicators in offshore Cyprus and Lebanon

Transcription

1 GeoArabia, v. 14, no. 2, 2009, p Gulf PetroLink, Bahrain Possible seismic hydrocarbon indicators in offshore Cyprus and Lebanon Per Helge Semb ABSTRACT A multi-client 2-D seismic survey, consisting of 6,770 line-kilometer of 2-D data, was acquired by Norway s PGS Geophysical offshore Cyprus in It was followed in 2006 and 2007 by the acquisition of two 3-D surveys, located offshore Lebanon (1,500 square kilometers), and offshore Lebanon and Cyprus (1,300 square kilometers). Further, in 2008 an additional 2-D survey was acquired offshore Cyprus to fill in the existing 2006 seismic 2-D grid. The D survey consist of 6,500 linekilometer and was acquired with the PGS dual sensor GeoStreamer technology (The D data is still being processed so no GeoStreamer examples are shown in this article). Since 2007, some of the deep-water areas (1,500 2,000 meters deep) offshore Cyprus have been offered for exploration and producing bid licenses by the government, while the Lebanese government is in the stage of preparing its offshore region for exploration and licensing. In this paper, several representative seismic lines are shown to illustrate the quality of the seismic data over this large region. The data shows that a Pliocene to Recent succession overlies a thick Upper Miocene evaporite section, known as the Messinian Salt. The blanketing salt-dominated interval is up to one kilometer thick and provides a regional seal over the area; although some faults are known to penetrate the salt and may allow seepage to occur. The pre-messinian Salt section is highly structured, with both horsts-and-grabens as well as large folds. In particular, several examples of seismic reflections are presented, which are interpreted as possible Direct Hydrocarbon Indicators (DHI). Some four-way structural closures are highly prospective as they are characterized by multiple DHIs; for example, a bright spot at the base of the Messinian Salt overlying a flat spot. Other examples consist of phase changes, dim spots, velocity pull-downs, low-frequency shadow zones and gas chimneys. Also shown are how some of these interpreted DHIs can be characterized in the 3-D surveys. The 3-D example illustrate a correlation between pre-messinian four-way structural closure and the root-mean square amplitude of the base Messinian Salt reflection. INTRODUCTION The offshore regions of Cyprus and Lebanon in the easternmost (Figure 1, Bremen, 2006, in Roberts and Peace, 2007; Elias, 2007) are currently unexplored for petroleum resources but this is set to change in the near future. Since 2007, Cyprus has conducted bid rounds for their offshore exploration and producing blocks (Figure 2), which are located close to the proven hydrocarbonproducing provinces of Egypt s offshore Nile Delta Cone and near-offshore basins of the Levantine in the southeastern. These deep-water basin areas, which contain a Mesozoic Cenozoic sedimentary succession up to 10 km thick, are prospective with large structures, a regional seal and an active petroleum system (see review in Roberts and Peace, 2007). According to Oil and Gas International (Oil and Gas International, Cairo, January 26, 2007) previous studies have been encouraging, estimating that oil and natural gas reserves in the seas surrounding Cyprus amount to 6 to 8 billion barrels of oil equivalent. It is therefore not surprising that the area is already receiving attention from the oil industry. This paper shows examples of recently acquired high-quality seismic data that clearly image several large structures, with associated potential Direct Hydrocarbon Indicators (DHI), which supports the prospective evaluation of the region. 49

2 Semb Isparta 31 Angle Adana 34 en de ru n 30 Ba sin STRUCTURAL ELEMENTS OF THE EAST MEDITERRANEAN Is k TURKEY Cilicia 100 km st hru sis T Kyrenia-Mi 35 CYPRUS Sout Strike-slip fault ru in Palmyra mp s re Slu sin a tb rth No Caspian Sea TURKEY N SYRIA Location Map Med. Sea IRAN km IRAQ JORDAN 29 KUWAIT BAHRAIN 41 QATAR EGYPT Arabian Shield UAE 30 Sea Tr AFRICAN PLATE 32 JORDAN EGYPT 33 Dead Nile River 31 LATE ansfo LEVA an NT P st or iya H Gindi Si na Nile Delta q radi Ghain u b A Bas 32 i-s yr ian F Slump ine ARABIAN PLATE old Be lt he es Pl Slump Ba rd il L 33 (Sy rian Arc) rn fs of e t es W aw 34 g Hi ho AFRICA 35 u hr st Levantine Nile Delta Cone 30 SYRIA LEBANON s ne he unt t os o at m Er Sea Katt Aleppo High e g Rid Hekateus Ridge h 32 u Tart Cyprus Bas 33 ia tak La t rus sin Ba h nc re Herodotus Reverse fault Coastal line and river 34 h Cyprus T ta La s Th Cyp Normal fault kia st Anim a Sea xander mou nts s rm N Figure 1: Tectonic provinces and prospective basins of the East Mediterranean (after Roberts and Peace, 2007; Elias et al., 2007). The main exploration provinces discussed in this paper are in the Cyprus, Herodotus, Latakia and Levantine basins. These basins are separated by thrust and transpressional zones associated with the convergence of the African, Levant and Arabian plates against the Turkish (Anatolian) Plate. OMAN SAUDI ARABIA SUDAN Red Sea YEMEN ERITREA Arabian Sea 50

3 Seismic hydrocarbon indicators, offshore Cyprus and Lebanon 35 0 N 70 km CYPRUS Block 1 Block 4 Block 5 North 3-D Block 3 Block 6 Block 7 Block 8 Block 2 Block Block 9 13 LEBANON South 3-D Block 10 Block 11 Block Figure 2: Exploration blocks 1 to 13 in offshore Cyprus, Mediterranian Sea, are shown together with the location of the 2006 PGS multi-client 2-D seismic survey in the offshore of Cyprus. Two 3-D surveys were also acquired in 2006 and 2007 in offshore Cyprus and Lebanon. DATA ACQUISITION AND PROCESSING Norway s PGS Geophysical acquired a multi-client 2-D survey (Figure 2), consisting of 6,770 linekilometer of 2-D seismic, in 2006 in offshore Cyprus. The survey was acquired with the vessel M/V Falcon Explorer using a 4720 cubic inch air gun at 8 meter and an 8,100 meter long streamer with 9.2 seconds recording length. The acquisition geometry and processing flow was optimized to image beneath the salt as well as image the Eratosthenes seamount (Figure 1), which has a different geology than the Cyprus Thrust Zone and the Levantine (Robert and Peace, 2007). The main seismic processing steps were: (1) minimum phase conversion; (2) attenuation of swell noise; (3) spatial anti-alias filter; (4) first-pass demultiple using Surface Related Multiple Attenuation (SRME); (5) Radon demultiple deconvolution; (6) 2-D Kirchhoff pre-stack time migration (PSTM); (7) residual moveout correction; and (8) post-stack processing. In addition, PGS acquired two multi-client 3-D surveys in 2006 and 2007 in offshore Cyprus and Lebanon waters (1,500 square km in the Levantine, and 1,300 square km in the Cyprus, Figure 2). The vessel M/V Atlantic Explorer used six streamers and two 3,090 cubic inch air guns positioned at a depth of 6 m. The streamer length was 6,000 m and the streamer separation 100 m. The processing flow was optimized to image below the salt with focus on true-amplitude preservation. The main processing steps included: (1) linear noise attenuation; (2) velocity analysis (initial velocities); (3) first-pass demultiple (SRME); (4) second-pass demultiple (Radon deconvolution); (5) zero-phasing and Q-compensation; (6) velocity analysis (migration velocities); (7) 3-D Kirchhoff prestack time migration (PSTM); (8) third-pass demultiple (Radon deconvolution); and (9) stacking and post-stack processing. 51

4 Semb EXAMPLES OF REGIONAL LINES Figure 3 and 4 show two seismic lines that illustrate the general quality of the seismic 3-D data. Northsouth Line A (Figure 3) was acquired in water depths of approximately 1,700 m and is perpendicular to the Lebanese coastline, about km offshore Beirut. Beneath the ocean bottom, a continuous Pliocene Recent sedimentary package overlies the Mesinnian Salt sequence. The package thickens towards Lebanon and blankets most of the Levantine. This sequence is believed to consist of turbidities that were sourced from mountains in Lebanon (Beydoun, 1977). Below the package, the Messinian evaporite sequence (Messinain Salt) is about one kilometer thick and dominated by rock salt. It can be mapped together with some more-or-less continuous intra-messinian packages, which are interpreted as reflections from gypsum, anhydrite and clastic beds. The Messinian Salt provides a regional seal in the region, below which large four-way-closure structures are evident in the Tertiary and Cretaceous section. One such structure is imaged to the left in Line A (Figure 3). Below the Messinian Salt, normal faults segment the section into horsts and grabens that may contain hydrocarbon traps. Many of the faults terminate below the Messinian Salt implying an extensional event occurred in the Early Tertiary. Other faults, however, also appear to continue through the salt and into the Pliocene Recent succession, seen as breaks both in the intra-messinian reflections and Pliocene Recent succession (e.g. Line A southern part and Line B western part, Figure 4). Indeed an anonymous reviewer noted that often large-scale, deep-seated faults penetrate right through the Messinian Salt and to the seabed, some with associated DHI and seepages evident. There are likely other shale seals as well. Other faults just penetrate from seabed down through the salt in which they usually sole out, at or around base salt. Further down in the seismic section, a reflection is here interpreted as the Senonian unconformity (i.e. top Jurassic). According to an anonymous reviewer this pick is highly debated by most interpreters because no direct well control is available. He reported that some interpreters have picked it as high as their intra-tertiary and as low as my base Cretaceous pick. West east Line B was acquired further to the north covering partly Cyprus and partly Lebanon (Figure 4). It was extracted from the middle of the northern 3-D survey, which was recorded in water depths of approximately 1,500 m. In contrast to Line A (Figure 3), Line B images extensive folds originating from the deep pre-cretaceous section. These may have started to form in the Late Cretaceous (Turonian) with the emplacement of the Todroos Ophiolite and related thrust systems of Cyprus and persisted until the Recent. The extensively folded zone, named the Latakia Ridge, pierces the sea bottom and stands about 400 m above it. It continues to the west where it connects to the Cyprus Thrust Zone (also known as the Larnaka-Latakia Ridge System) along the southern part of Cyprus. This zone accomodates subduction related to the collision between the north-drifting Africa, Arabian and Levant plates and the Turkish (Anatolian) Plate. The Latakia Ridge is a morphological high feature that formed by a combination of a subduction and left-lateral, strike-slip movements. DIRECT HYDROCARBON INDICATORS Direct Hydrocarbon Indicators (DHI) have been successfully used to identify the occurrence of hydrocarbons from reflection seismic data for over 30 years. The methods originated primarily during the exploration of Upper Tertiary clastic reservoirs. It has since evolved into a tool that can be used in virtually every geological province (Rice et al., 1981). The most commonly identified DHIs include flat spots, bright spots, dim spots, phase changes, velocity push-downs, low-frequency zones, gas chimneys and shallow high-amplitude zones. DHIs provide one of the simplest methods for identifying prospects, especially when they occur together with seismically defined structural and stratigraphic traps. Even though DHIs are widely used, great caution is required when analyzing a prospect. An event that appears as a DHI might be due to an abrupt lateral lithological variation, diagenetic effects, large pressure change or unreliable seismic amplitude information. In cases where a dry hole cannot be 52

5 Seismic hydrocarbon indicators, offshore Cyprus and Lebanon ascribed to any of these causes, it is often due to low gas saturation (O Brien, 2004). An anonymous reviewer noted correctly that: even a small change (a few percentages in gas presence) can produce a DHI effect, which is just a gas show but not a commercial discovery. Unfortunately the quality of seismic does not always improve the quality of DHI reliability. It is therefore important to evaluate the geological risk by also considering the geological setting of the DHI to assess whether an amplitude anomaly might represent hydrocarbons (Roden et al., 2005). DHI prospects may be interpreted and assessed differently from company to company. For additional proof of hydrocarbon presence, a DHI prospect should be assessed by combining information from other analyses, e.g. amplitude-versus offset (AVO) studies. Seismic Flat Spot A seismic flat spot may be a reflection from a well-defined fluid contact, commonly the gas/oil or gas/ water contacts. The acoustic impedance contrast between the two phases may be sufficiently large to produce a strong reflection. In a section with dipping reflections, it stands-out because of its flat attitude. This is usually taken to be the most definitive and informative of all the Direct Hydrocarbon Indicators (Sheriff, 1995). A good example is the ultra-deep water Shell gas discovery in Well Kg 451b in Egypt s offshore Nile Delta NEMED block, where a flat spot was identified in the Pliocene, as clearly imaged in the 2-D seismic data (Figure 5). In offshore Cyprus, flat spots occur in the Cretaceous and Tertiary successions in several areas: (1) to the northwest of the Eratosthenes Seamount; (2) south of the Cyprus Thrust Zone; and (3) below the Messinian Salt in the Levantine. In offshore Lebanon, flat spots have been identified from 2-D and 3-D seismic surveys in Cretaceous and Tertiary sequences in the Levantine. In Figure 6 an example of a Pliocene flat spot and two bright spots are shown from offshore Cyprus in the western part in the Herodotus (Figure 1). These DHIs are believed to occur in the same stratigraphic interval as in Shell s discovery but were not directly tied to Shell s survey or wells in the Nile Delta cone (Figure 5). Figure 7 shows another example of a flat spot imaged below the Messinian Salt and above a large structural closure. Seismic Bright Spot A seismic bright spot is a strong-amplitude reflection caused by large changes in acoustic impedance and tuning effects. In general, bright spots are mostly caused by lateral changes in lithology rather than DHIs. Nevertheless bright spot DHIs can also be due to a gas-saturated sandstone reservoir underlying a shale interval (Figure 5). When seismic bright spots are on top of a structural high they are often associated with gas accumulation as implied in Figure 6. Gas-induced bright spots usually have negative polarity for the reflection from the top of the reservoir. An anonymous reviewer added: Bright spots may have several causes and no general rule is likely to apply. In siliciclastic rocks with intermediate to high porosity (15 30%), bright spots may be caused by gas in both good and poor reservoirs, with high or low gas saturations. In this setting, low gas saturations (5 10%) may give a similar response compared to a case with high saturation, in addition often also giving erroneous positive results in the AVO analysis. In carbonates and low porosity siliciclasics, bright spots are more likely to reflect lithological changes, primarily porosity. Seismic Dim Spot By contrast to a bright spot, a seismic dim spot shows weak rather than strong amplitudes. The weak amplitude can correlate to the presence of hydrocarbons that reduce the contrast in acoustic impedance (AI) between the reservoir and the overlying rock. Such dim spots are often associated with the occurrence of oil or gas. 53

6 54 Senonian Unconformity Eocene Unconformity Base Miocene Base Middle Miocene Base Upper Miocene Messinian Salt PlioceneRecent LINE A, SOUTH 3-D, LEVANTINE BASIN 10 km South Figure 3a: NS-oriented seismic Line A extracted from multi-client PGS 3-D survey acquired in offshore Lebanon in the Levantine (South 3-D in Figure 2). The water depth is approximately 1,700 m. North Semb Two-way Time (second)

7 55 Senonian Unconformity Eocene Unconformity Base Miocene Base Middle Miocene Base Upper Miocene Messinian Salt PlioceneRecent LINE A, SOUTH 3-D, LEVANTINE BASIN 10 km South Figure 3b: Interpretation of Line A shows the thick (c. 1,000 m) Messinian Salt below which numerous normal faults fragment the section into a horst-and-graben structural style. Deeper reflections are approximately interpreted without well control. North Seismic hydrocarbon indicators, offshore Cyprus and Lebanon Two-way Time (second)

8 Two-way Time (second) LINE B, NORTH 3-D, CYPRUS AND LEBANON Latakia Ridge 10 km East Figure 4a: EW-oriented seismic Line B extracted from multi-client PGS 3-D (North 3-D acquired partly in Block 3 of offshore Cyprus and offshore Lebanon, see Figure 2). The water depth is approximately 1,500 m and the Latakia Ridge rises about 400 m above the sea bed. West Senonian or older Top Eocene? Top Oligocene Messinian Salt PlioceneRecent Semb

9 Two-way Time (second) LINE B, NORTH 3-D, CYPRUS AND LEBANON Latakia Ridge 10 km East Figure 4b: Interpretation of Line B shows the Messinian Salt is regionally thinner than in Line A (Figure 3) and absent over the Latakia Ridge. In contrast to EW-oriented Line A, Line B shows high-amplitude folds due to compression. West Senonian or older Top Eocene? Top Oligocene Messinian Salt PlioceneRecent Seismic hydrocarbon indicators, offshore Cyprus and Lebanon